It runs on an existing Canadian Pacific Railway track (Ellwood and Prescott subdivisions of Bytown and Prescott Railway[17]), so the only construction work necessary was to build the stations themselves and the passing tracks necessary to allow trains to operate in both directions.

The track is shared with Ottawa Central freight trains south of Walkley Yard, which occasionally serve the National Research Council of Canada's Automotive and Surface Transportation Research Centre From 2001 until 2015,[18] the system used three diesel-powered Bombardier Talent BR643 low-floor diesel multiple unit trains.

It was, however, described as "light rail", partly because plans called for it to be extended into Ottawa's downtown as a tramway-like service, and partly because the Talent vehicles, though designed for mainline railways in Europe, are much smaller and lighter than most mainline trains in North America, and do not meet the Association of American Railroads' standards for crash strength.

Ottawa is also authorized to run trains with only a single operator and no other crew, something rare on mainline railways in North America.

Tickets can be purchased from a vending machine on the platform, and certain bus passes are also valid for the Trillium Line.

In order to enable night-time use of the line by standard-width freight services, retractable platform extenders are mounted at each station (other than Bayview which is constructed on its own private rail spur).

[22] On January 16, 2003, the Ontario chapter of the American Public Works Association (APWA) presented the City of Ottawa, Canadian Pacific Railway and Morrison Hershfield with the APWA Public Works Project of the Year award in the transportation category.

The other criticism is that there is very low ridership of the trains compared to some very crowded bus lines such as the 90–99 series routes.

The project would have terminated diesel light rail service on the Trillium Line so as to reuse its right-of-way for a double-track, electric light rail line that would have extended west from the University of Ottawa to Bayview then south to Leitrim and then west to Barrhaven.

However, in December 2006, Ottawa City Council cancelled this project, thus leaving the diesel-powered Trillium Line unchanged.

[10] Six new Alstom Coradia LINT 41 trainsets (replacing the three older Bombardier Talent units) and the two extra passing loops allowed the number of trains on the line to double to four.

[26] After the completion of the Stage 2 project, the line was originally planned to continue operating using single-car trains; however, because of the lower-than-intended frequency, the city was forced to adjust the Stage 2 plan to include longer trains and platforms to compensate.

[31] The project also included a number of other significant upgrades including the lengthening of all existing passing tracks, the purchase of seven new trains, the doubling in length of all existing platforms, the grade separation of the Ellwood diamond to allow Via trains to cross underneath the Trillium Line, the rehabilitation of rail bridges over the Rideau River and of the Dow's Lake rail tunnel, the upgrading of the signalling system to allow the implementation of positive train control, the construction of several new pedestrian tunnels and overpasses, and numerous guideway and vehicle rehabilitation projects.

[32] The contract for the project was approved on March 7, 2019, by city council,[33] with construction of the airport spur beginning in mid-2019.

[36] On February 22, 2019, the city announced that the selected proponent to construct the project was TransitNEXT, a wholly owned subsidiary of SNC-Lavalin.

[33] It was later revealed that TransitNEXT's bid had not met the minimum technical scoring threshold in order to be considered, which continued the controversy.

[40] An investigation conducted by the city's auditor general later confirmed that the authority delegated by council gave city staff sole discretion on whether to allow a bid to proceed even if it had not met the minimum scoring threshold, and that staff had otherwise correctly followed the entire procurement process that had been approved by council.

[42][43][needs update] On May 26, 2023, the Light Rail Transit Sub-Committee received an update on the progress of construction for Lines 2 and 4, noting that construction was going well and that handover of the system to the city was expected in approximately the first or second week of October 2023, though that may shift earlier or later pending the results of operational testing across both lines.

[49] On September 24, 2019, the cities of Ottawa and Gatineau jointly announced that they no longer intended to use the Chief William Commanda Bridge for any kind of rail connection, citing capacity concerns at Bayview station.

The Trillium Line initially used three Bombardier Talent diesel multiple unit (DMU) trains for service.

After being retired in 2015, the units were put up for auction multiple times[52][29] but a deal to sell the vehicles was never successfully established.

[56] On May 3, 2018, it was announced that the city would be purchasing seven new Stadler FLIRT trains to use on the extended Trillium Line after the completion of Stage 2.