An airport rail link was one of the priority projects identified in Metrolinx's regional transportation plan, The Big Move.

[10] The UP Express has been criticized for not using electric trains from the outset, and for its initial fare prices that were very high compared with other North American airport–city connections.

On 13 November 2003, the Union Pearson AirLink Group, a subsidiary of SNC-Lavalin, was selected to finance, design, construct, operate and maintain an airport rail link.

In June 2008, the Ontario government revised the environmental assessment process from a required three-year period to six months, and later that summer it resumed negotiations with Union Pearson AirLink Group on terms for a public-private partnership.

Furthermore, the high-speed link received public statements of strong support from Premier of Ontario Dalton McGuinty, Toronto Mayor David Miller and the federal government.

[15] In December 2008, Metrolinx became the proponent of new EA that would simultaneously assess a combination of both the UP Express and GO Transit's Georgetown South expansion.

[15] After two years of failed negotiations and financing difficulties, the public-private partnership between Ontario and the Union Pearson AirLink Group was called off in July 2010, and Metrolinx was handed responsibility to directly own and operate the service.

[3] Metrolinx largely preserved the same project scope as had been approved in the environmental assessment, with the service designed for air travellers rather than for conventional commuters.

[28] Undertaken as part of the Georgetown South Project, the work consisted of five new road overpasses and two road underpasses, one railway grade separation, one bridge reconstruction and the widening of 15 other bridges, construction or re-construction of four stations, major track and grading construction, signal installations utility relocations, as well as the new 3.3-kilometre (2.1 mi) elevated spur to and from the airport.

President Kathy Haley, who was heading up the project, stepped down on 31 March 2016, in a shakeup of the ranks that resulted in changes to the line's management approach.

This 22-kilometre (14-mile) section of track between Bathurst Street and Highway 427 was upgraded as part of GO's Georgetown South Project, which improved infrastructure along the route to accommodate the UP Express, GO Transit, Canadian National, and Via Rail services.

[37][38] The elevated spur to Pearson Airport reaches a maximum height of 28 metres (92 feet), offering a view of the Downtown Toronto and Mississauga City Centre skylines.

The platform is fully enclosed and features a waiting area, a guest services desk, café, gift shop, and lounge.

[42] Initially, on 1 March 2011, Metrolinx announced that it had chosen to buy 12 DMU cars (six two-car trains) from Nippon Sharyo at a cost of $53 million, for the service.

[28][42] The DMUs were manufactured in Japan, assembled in Nippon Sharyo's facility in Rochelle, Illinois, and towed by rail to Toronto, with the first trainset arriving on 15 August 2014.

[39] Braking energy is converted into electricity by the auxiliary power generator, and helps to provide onboard lighting and heating.

[49] The 2010 Metrolinx electrification study, which did assume the use of Tier 4 emissions technologies for diesel propulsion options, found that the Georgetown–UP Express diesel service would contribute on the order of 0.2% to local air pollution, and that electrification would result in only a small improvement to local air quality.

[50] In an additional review of human health assessment in 2011, GO Transit examined the current plan for Tier 4 emission controls on both the UP Express trains and conversion of existing GO locomotives to Tier 4 and found that the human health risks of the diesel UP Express service are negligible, but that current background air quality (from other pollution sources in Toronto) is an issue.

[67] On 11 March 2021, UP Express became the first transit agency in the Greater Toronto Area to launch the new Presto open payment system.

[68][69] Residents living along the Weston section of the route have objected to alterations to accommodate the GO Georgetown South corridor improvements and the addition of UP Express trains.

"We believe the Greater Toronto Area needs this project as fast as possible, and that means going with the cleanest diesel technology in the world," stated Prichard.

The application requested "quashing and setting aside any decision to implement or run Diesel Multiple Units along the Air Rail Link", on the basis that by taking direction from the Ontario government to complete the UP Express in time for the Pan American Games, a proper analysis between operating diesel and electric trains was not completed, running contrary to Metrolinx's legislated mandate.

It also asserted that another review of air quality was warranted due to the World Health Organization's (WHO) reclassification of diesel exhaust as a carcinogen.

[76] As of 2019[update], Metrolinx is planning a major upgrade to the Union Pearson Express to better integrate it into the GO Transit rail network and to double the frequency of trains to the airport.

The proposal includes:[77][78] According to the planning document dated 22 February 2019, it is uncertain whether the UP Express infrastructure at airport Terminal 1 can be modified to accommodate electrified GO trains.