GO Transit riders heading in and out of Mississauga were dealing with a third day of disrupted service on Wednesday after a train derailed near Union Station in Toronto on Monday morning.

Metrolinx, the provincial agency that runs GO Transit, said a slow-moving train leaving Union came off the track and struck a track switch. The incident caused signal problems that limited the movement of other trains, leading to major delays at the country's busiest train station during the Monday morning rush.

All three GO rail lines serving Mississauga, Lakeshore West, Milton and Kitchener, run into Union, so problems there ripple out to stations across the city.

UP Express to Pearson paused

The disruption also stopped the Union Pearson Express for part of Monday morning, cutting the rail link between downtown Toronto and Toronto Pearson International Airport in Mississauga. Express buses ran between the Union Station Bus Terminal and Pearson's Terminal 1 while trains were out of service. UP Express trains returned shortly before 12:30 p.m., though officials warned of longer-than-normal waits.

Modified schedule continues

Metrolinx CEO Michael Lindsay said the derailed train was put back on the tracks late Monday. GO ran a modified schedule on Tuesday and was still running one on Wednesday as crews continued service recovery work near Union. Metrolinx advised riders to check schedules online before travelling and to allow extra time. Staff were posted at Union Station to help passengers.

We know it's been a challenging commute and we appreciate your continued patience as our teams work to safely return service to normal.

— GO Transit service notice

Lakeshore West weekend closures wrap up

There was better news for Lakeshore West riders. Metrolinx said crews have finished replacing the century-old Etobicoke Creek railway bridge just west of Long Branch GO station, on the Toronto-Mississauga border.

The work had forced full-weekend closures on part of the line over several months, including three weekends in December and the Jan. 17-18 and Jan. 24-25 weekends. In January, crews replaced the structures supporting the third track over the creek and removed the large crane used to lift the new bridge into place. Metrolinx says the new bridge supports future expansion of the corridor for more frequent service.