For MAX, a "line" refers to the physical railroad tracks and stations a train serves within its designated termini, i.e. a train "route" or "service". MAX operates five lines, each assigned a color. TriMet adopted the use of colors to distinguish separately operated routes in 2000 and brought them into use on September 10, 2001, when it opened the Airport MAX extension. On that day, the service running between Hillsboro and Gresham became designated the Blue Line, while that running between downtown Portland and Portland International Airport was designated the Red Line.

Every MAX line interlines with at least one other service, particularly as it approaches the system's central area. The Steel Bridge accommodates the most interline routes with four lines (Blue, Green, Red, and Yellow) utilizing the same tracks. TriMet has modified train routes over time, often as part of system expansions. For example, the Yellow Line, which began service in 2004, originally followed the same route into downtown Portland as the Blue and Red lines. It was realigned to the transit mall in 2009 when light rail service was introduced to that corridor.

List of MAX lines

Service	Stations	Termini
Blue Line	48	Hatfield Government Center (Hillsboro)	Cleveland Avenue (Gresham)
Red Line	37	Hillsboro Airport/Fairgrounds	Portland International Airport
Green Line	30	PSU South	Clackamas Town Center Transit Center
Yellow Line	17	Expo Center	PSU South
Orange Line	17	Union Station	Southeast Park Avenue (Milwaukie)

MAX operates on a mixture of shared and exclusive transit right-of-way. Within downtown Portland, trains run on surface streets. They operate in dedicated lanes restricted to personal vehicles, and operators abide by the city's traffic control system. On the Morrison and Yamhill couplet, MAX travels in the left lanes. On the Portland Transit Mall (5th and 6th couplet), MAX shares dedicated lanes with buses; both vehicle types travel in the center or right lanes and stop at their respective curbside platforms on the right lane. Lanes may be separated by turtleback delineators or double-solid white lines, and marked with white diamonds or white "T"s.

Outside of downtown Portland, MAX runs on street medians and viaducts, alongside freeways and freight lines, and underground. Where the tracks run within a street median, intersections are controlled by traffic signals that give trains preemption. Where tracks run on a separate right-of-way, trains are protected by automated grade crossing gates when traversing level crossings. Some segments of MAX are elevated to carry trains over busy thoroughfares and difficult terrain. A three-mile (4.8 km) section of tracks runs beneath Washington Park in Portland's West Hills through the Robertson Tunnel, the system's longest underground segment.

MAX crosses the Willamette River using the Steel Bridge and Tilikum Crossing. In studies conducted for the Eastside MAX, planners recommended using the Steel Bridge due to its former role as a river crossing for the city's historic streetcars. When MAX commenced service in 1986, trains shared the bridge's center lanes with vehicular traffic. In 2008, workers closed the bridge's upper deck to construct a junction between the Eastside MAX tracks and the newer Portland Transit Mall tracks. Upon reopening, the two inner lanes became exclusive to MAX trains, while cars, buses, and other motorized traffic were restricted to the two outer lanes. TriMet designed and built the newer Tilikum Crossing to accommodate transit vehicles (MAX, streetcar, and buses), cyclists, and pedestrians only; with the exception of emergency responders, private vehicles are prohibited. Tilikum Crossing is thus recognized as the first major "car-free" bridge in the United States.

MAX is powered by a conventional 750-volt direct current (DC) overhead wire system. Most of the system uses a dual-wire catenary, with a contact wire supported by a messenger wire. In central city areas such as downtown Portland, however, it uses a single contact wire to minimize the amount of overhead wiring. To further minimize visual impact, ornamental street light poles, buildings, and bridge structures are used to support the wiring. Substations, spaced approximately every one mile (1.6 km) apart, convert the high-voltage public supply to the voltage power used by trains. The power system can bridge any one substation so that trains can continue to operate should a substation or its supply go down.

Approximately 70 percent of the MAX system uses automatic block signaling (ABS), which allows for relatively fast operating speeds—up to 55 miles per hour (89 km/h)—and short headways. For example, between Lloyd Center/Northeast 11th Avenue station and Gateway Transit Center along the Banfield Freeway, ABS can accommodate an operating headway of two minutes. Within these sections, automatic train stops (ATS) enforce speed limits and automatically apply the brakes should a train operator fail to do so. The remaining 30 percent of the system relies on traffic signals and line-of-sight operation. Speeds do not exceed 35 mph (56 km/h) in these sections.

TriMet's vehicle-maintenance complexes for the MAX system are the Ruby Junction facility in Gresham and the smaller Elmonica facility in Beaverton. The Ruby Junction facility is located near Ruby Junction/East 197th Avenue station while the Elmonica facility is adjacent to Elmonica/Southwest 170th Avenue station; both are on the Blue Line.

Ruby Junction began with one building that TriMet built as part of the original MAX project in the early 1980s; it had expanded to three multi-story buildings totaling 143,000 square feet (13,300 m) occupying 17 acres (6.9 ha) by 2010, and to four buildings totalling 149,000 square feet (13,800 m) occupying 23 acres (9.3 ha) by 2016. It contains 13 maintenance bays and its yard tracks have the capacity to store 87 light rail cars. In 2016, around 200 employees worked at Ruby Junction and almost 200 MAX operators operated trains that were based there. In addition to vehicle maintenance, crews who maintain the MAX system's tracks and signals are also based at Ruby Junction. In 2015, some maintenance-of-way personnel moved into the Portland Vintage Trolley carbarn next to Rose Quarter Transit Center after Vintage Trolley service was discontinued.

Ruby Junction originally housed light-rail operations, communications, and administrative workers.

The Elmonica facility was built as part of the Westside MAX Project in the mid-1990s and was completed in 1996. Its building has 78,000 square feet (7,200 m) of space.

MAX consists of 94 stations, of which 48 are served by the Blue Line, 30 by the Green Line, 37 by the Red Line, 17 by the Orange Line, and 17 by the Yellow Line. Furthermore, 47 stations are served by at least two lines and eight stations are served by three lines. The system's central stations, where all MAX services interconnect, border the two city blocks in downtown Portland occupied by the Pioneer Courthouse and Pioneer Courthouse Square; they are the Pioneer Courthouse and Pioneer Place stations—served by the Green, Orange, and Yellow lines—and the Pioneer Square stations—served by the Blue and Red lines.

MAX stations vary in size but are generally simple and austere. Platforms are about 200 feet (61 m) long as a result of Portland's short city blocks in downtown, which restrict trains to two-car consists. Like other North American light rail systems, MAX stations do not have faregates; paid fare zones are delineated but remain accessible to anyone. In 2015, TriMet proposed installing turnstiles at some stations along the Portland–Milwaukie segment but never did so. Stations are typically equipped with trash cans, shelters, and ticket vending machines. Most stations have arrival information displays that show when trains arrive and other service information. These displays were first installed at I-205 and Portland Transit Mall stations, and a federal grant in 2013 enabled TriMet to add more at other locations. Concessionaires sometimes open coffee shops at certain stations.

A majority of MAX stations are at street level, correlating to the system's predominant alignment. Sunset Transit Center, Southeast Bybee Boulevard, and stations along the Banfield Freeway are below street level. One station, Lents Town Center/Southeast Foster Road, is elevated. Washington Park is the system's only underground station and holds the distinction as North America's deepest transit station at 260 feet (79 m) below ground.

Many MAX stations facilitate transfers to other modes of public transit. 11 stations are transit centers with connections to multiple local and intercity bus routes. Beaverton Transit Center is the only MAX-served transit center with a transfer to the region's commuter rail line, WES Commuter Rail, which operates between Beaverton and Wilsonville in Washington County. Within the Portland Transit Mall, trains connect with buses serving downtown Portland; bus stops take up transit mall blocks unoccupied by light rail platforms. MAX riders can transfer to the Portland Streetcar at points where MAX and streetcar lines intersect and to Amtrak via two stations near Portland Union Station. The Red Line operates as an airport rail link with a stop at a MAX station attached to the main passenger terminal of Portland International Airport.

TriMet has built a total of six infill stations. Four were built on the original Eastside MAX alignment—Mall/Southwest 4th Avenue (1990), Mall/Southwest 5th Avenue (1990), Convention Center (1990), and Civic Drive (2010)—while two were built on the Portland Transit Mall—PSU South/Southwest 6th and College (2012) and PSU South/Southwest 5th and Jackson (2012). On March 1, 2020, TriMet permanently closed the Mall infill stations in an effort to speed up travel times in downtown Portland. The agency also temporarily closed Kings Hill/Southwest Salmon Street station for a trial period ending on March 1, 2021.

Stations built as part of the Banfield Light Rail Project were originally fitted with electric wayside lifts to accommodate riders with mobility devices on the system's high-floor, first-generation vehicles. Each station had two lifts, one for each direction of travel. The lifts were installed on platforms rather than on trains to prevent malfunctions from delaying service. Increased use of the lifts eventually became the cause of delays, and many users felt stigmatized by the lifts' "box" design and time-consuming operation. After the Americans with Disabilities Act (ADA) was signed into law, TriMet developed a paratransit plan in January 1992. Just before the start of the Westside MAX Project, MAX became the first light rail system in North America to procure low-floor vehicles after a TriMet study of European systems. The low-floor cars, which TriMet and Siemens jointly developed, entered service in August 1997.

MAX achieved full accessibility in April 1999. Ticket vending machines provide information and instructions in audio, braille, and raised lettering. Station platforms also have signs with braille and raised lettering to indicate which lines provide service and where they go. The edge of platforms have tactile paving to warn riders from standing too close to the edge. Non-street-level platforms may be accessed with elevators. Most light rail cars, with the exception of Type 1, are low-floor and have ramps that extend onto platforms to allow mobility devices to board. High-floor Type 1 cars are paired with low-floor Type 2 or 3 cars to maintain accessibility. In each train, an audio system and LED signs announce the name of each upcoming station. All trains have spaces and priority seating areas reserved for seniors and people with disabilities, and service animals are permitted on board.

In 2011, TriMet began upgrading the oldest sections of MAX to improve pedestrian safety and compliance with updated ADA standards. TriMet installed pipe barriers at Gateway Transit Center platform crossings to force pedestrians to slow down and face oncoming trains before crossing the tracks and realigned sidewalks and crosswalks at four at-grade crossings in Gresham. Other improvements made throughout the line include the installation of pedestrian warning signals and tactile paving upgrades.

Based on a report published in 2019, TriMet provides a total of 12,614 park-and-ride spaces, of which 10,219 directly serve 25 MAX stations. The agency's parking facilities are either surface lots or multi-level garages, and they are free to use. TriMet allows vehicles to park at most stalls overnight as long as they do not exceed 24 hours. At some locations, TriMet negotiates with nearby establishments for additional parking spaces. Westside MAX stations contain 3,643 parking spaces, the most number of spaces in a corridor. Clackamas Town Center Transit Center on the I-205 MAX segment includes a 750-space parking garage, the largest capacity of any single MAX station. Southeast Holgate Boulevard station, also on the I-205 MAX, provides the fewest parking spaces with 125 stalls.

In the 2019 report, passengers originating from TriMet park and rides accounted for five percent of TriMet's total weekday ridership. In 2017, the Portland–Milwaukie segment had a 100-percent usage rate of its available spaces while the Westside MAX segment had 85 percent. The corridor with the lowest use of available parking spaces was the I-205 MAX at 30 percent; TriMet attributes this to factors such as inconvenient lot access and the Green Line's indirect route to downtown Portland compared with the availability of more direct bus routes. The cost-per-space for building park and rides is estimated at $18,000 per surface-lot space and $52,000 per structured space.

TriMet additionally offers four different bicycle parking options at its MAX stations, although not all options are available at every station. Bike and rides are secure, enclosed spaces that are accessible by keycard and are monitored 24 hours per day by security cameras; as of 2020 they are available at eight stations. Electronic bicycle lockers, or eLockers, are secure lockers that may also be accessed by keycard and are made available on a first-come, first-served basis. TriMet contracts some keycard access to BikeLink and uses its Hop Fastpass on others. Other lockers may be rented by users. Bicycle racks are the most common form of bicycle parking.

As of 2020, TriMet operates five models of light rail vehicles designated as "Type 1" through "Type 5", of which two are successive upgrades of the same model. The MAX system's 145 cars vary in length, from 88 feet (26.8 m) to 95 feet (29.0 m), and are used interchangeably on every line. Downtown Portland's 200-foot (61 m) downtown blocks allow the operation of only one or two consists to prevent stopped trains from blocking intersections. Type 2 and 3 low-floor vehicles may run singularly or coupled to another Type 1, 2, or 3 vehicle. Type 1 high-floor vehicles are also capable of running singularly, but doing so would constrain accessibility due to a lack of wheelchair access. Thus, a high-floor car must be coupled with a low-floor car. Type 4 and 5 cars can only be coupled to one another.

Twenty-six Type 1 high-floor vehicles were produced for the Banfield light rail project by a joint venture between Bombardier and La Brugeoise et Nivelles beginning in 1983. TriMet announced it would purchase seven additional vehicles that August, but a budget shortfall forced the agency to withdraw this proposal the following November. The cars are similar in design to Bombardier vehicles that had been used in Rio de Janeiro. Bombardier built the frames in Quebec but its factory in Barre, Vermont, manufactured the majority of each car, the first of which arrived in Portland in 1984. Each 45-short-ton (41 t) car is single-articulated and contains six axles. The high floors connect with the low platforms through interior steps, which necessitated platform wheelchair lifts until the arrival of low-floor cars. A car sits 76 people and has an overall capacity of 166.

In 1992, TriMet officials conducted an accessibility study and determined that low-floor cars were the most cost-effective alternative to providing universal access. MAX then became the first light rail system in North America to acquire low-floor train sets when TriMet procured 39 model SD660 cars from Siemens in 1993. These Type 2 cars were equipped with doorway wheelchair ramps. They entered service during the partial opening of the Westside MAX in 1997. By 2000, TriMet had ordered 17 more Type 2 cars including six for the Airport MAX project. The system's 27 Type 3 vehicles, which the agency purchased as part of the Interstate MAX project and first brought into use in 2003, are the same model as the Type 2 vehicles but with technical upgrades and a new livery.

Twenty-two Siemens S70 low-floor cars, which were designated Type 4, were purchased in conjunction with the I-205 MAX and Portland Transit Mall projects, and were first used in 2009. Type 4 cars have a more streamlined design and more seating, and are lighter and more energy-efficient than the previous models. The Type 4 cars were the first in the MAX network to use LED-type destination signs. The second series of S70 cars, TriMet's Type 5 vehicles, were procured for the Portland–Milwaukie light rail project. TriMet placed an order for the Type 5 cars with Siemens in 2012 and delivery commenced in 2014. These vehicles include some improvements over the Type 4 cars, including less-cramped interior seating, and improvements to the air-conditioning system and wheelchair ramps. These introduced a new seating layout in the center section, among other changes, and Siemens later retroactively redesignated TriMet's Type 5 cars as model S700.

In July 2019, TriMet placed an order for 26 Siemens S700 light rail vehicles that are intended to replace the system's Type 1 vehicles. The order was expanded to 30 cars in June 2021. The first car was delivered in December 2022, and the type will be designated Type 6.

From Monday to Thursday, MAX trains run for 221⁄2 hours per day. Additional late-night trips are provided on Fridays. Except for additional late-night trips on Saturdays, weekend service runs on a slightly reduced schedule. TriMet designates all MAX lines as "Frequent Service" routes, which ensures service runs on a 15-minute headway for most of each day. During the early morning and late evening hours, trains operate with headways of up to 30 minutes. During rush hours, headways can be as short as three minutes, particularly in the central section of the system where lines overlap. At many stations, a live display shows the destination and time-to-arrival of the next several trains using data gathered by a vehicle tracking system installed on the light rail tracks.

MAX carried over 38.8 million total passengers in 2019, an average of 120,900 riders per day on weekdays. This is slightly lower than the number of riders recorded in 2018 and represents the system's third consecutive year of fallen ridership. MAX ridership peaked in 2012, when the system recorded around 42.2 million annual passengers. 2016 marks the last year ridership increased; this was due to the opening of the Orange Line. TriMet attributes falling ridership to perceived crime within trains and stations and lower-income riders being forced out of the inner city by rising housing prices. In 2019 (prior to the COVID-19 pandemic), MAX was the fourth-busiest light rail system in the United States after the light rail services of Metro Rail in Los Angeles, the MBTA in Boston, and Muni Metro in San Francisco.

As is standard practice on North American light rail systems, MAX uses proof-of-payment for fare collection, and stations do not have ticket barriers. TriMet employs an automated fare collection system through a stored-value, contactless smart card called Hop Fastpass, which can be purchased from the TriMet ticket office or participating retail outlets. Smartphone users may download a virtual version of Hop Fastpass, while single-use Hop Fastpass tickets are dispensed by ticket vending machines at every MAX station. Smartphones with a debit or credit card loaded into Google Pay, Samsung Pay, or Apple Pay, and Portland Streetcar 21⁄2-hour tickets and one-day passes can also be used to board MAX. Riders must tap their fare medium onto a card reader with each boarding. Fares are flat rate and are capped according to use. Using Hop Fastpass, riders may transfer to the Portland Streetcar and other TriMet and C-Tran services.

On August 25, 2024, TriMet introduced four new bus routes to replace late-night MAX services, to expand the length of time available each night for routine overnight maintenance. The new routes replace the last one or two MAX trips of the night on the Blue (two bus routes), Red, and Yellow Lines, but not the Green Line. This practice had already been in effect on the Orange Line since its opening in 2015.

From the MAX system's opening until 2012, riding trains within Fareless Square, which was known as the Free Rail Zone from 2010 to 2012, was free of charge. Fareless Square included all of downtown and, starting in 2001, part of the Lloyd District. The 37-year-old fare-free zone was discontinued on September 1, 2012, as part of system-wide cost-cutting measures. As part of the same budget cuts, TriMet discontinued its zonal fares and moved to a flat-fare system. Zones had been in place since 1986; higher fares were charged for longer journeys across four paid zones.

The MAX Mall Shuttle operated on weekday afternoons from when it was introduced on September 14, 2009, until 2011. It acted as a supplement to the light rail service provided on the Portland Transit Mall by the Green and Yellow lines. The Mall Shuttle operated between Union Station and Portland State University every 30 minutes from noon until 5:30 p.m. TriMet discontinued this supplementary shuttle service on June 5, 2011. Along with bus services, the mall continues to be served by two MAX lines in each direction—Green and Yellow lines northbound and Green and Orange lines southbound—which provide a combined average headway of 7.5 minutes in each direction at most times.

The Portland Vintage Trolley operated on the MAX system on most weekends from 1991 until 2014, serving the same stops. This service used 1991-built replicas of 1904 Portland streetcars. Originally, the Vintage Trolley service followed a section of the original MAX line between the Library and Galleria stations and Lloyd Center. In September 2009, the service moved to the newly opened MAX alignment along the transit mall, running between Union Station to Portland State University, and remained on this route in subsequent seasons. In 2011, the service was reduced to seven or eight Sundays per year, and in July 2014 it was discontinued entirely and the two remaining faux-vintage cars were sold to a group planning a streetcar line in St. Louis.

TriMet employs a transit police division to patrol MAX and other TriMet property. Most of its officers serve with local law enforcement agencies and are assigned terms with the transit police; this partnership with local police enables the closest available unit to respond to incidents. TriMet also partners with the Transportation Security Administration, which provides a canine unit. Riders are encouraged to alert TriMet employees using on-board intercoms or to dial 9-1-1 upon witnessing a crime or suspicious activity. TriMet operates over 4,000 security cameras; all MAX trains and stations became fully equipped with cameras in 2014.

On May 26, 2017, at approximately 4:30 pm, a man fatally stabbed two people and injured a third after he was confronted for shouting anti-Muslim slurs at two teenage girls inside a MAX train. Two men—a technician and U.S. Army veteran, and a recent university graduate—died from wounds to their necks while a third male victim survived. The attacker, who described himself as a white nationalist, was arrested and charged with murder, attempted murder, and other crimes. On February 21, 2020, the perpetrator was found guilty on all charges, including two counts of first-degree murder. This resulted in a mural being painted on the station entrance of the Hollywood Transit Center, where the stabbing occurred.

On November 15, 2023, shortly after 10:00am, a MAX train collided with a Portland Streetcar in the Lloyd District and injured two people on board.

• List of rail transit systems in the United States
• Transportation in Portland, Oregon

Wikimedia Commons has media related to MAX light rail.

• "MAX Light Rail". TriMet.