Track maintenance and platform upgrades at two GO stations means the Barrie GO line will be out of service from 9 p.m. Friday night until the end of service on Sunday.
Riders who were planning to take the GO train out of Barrie will have to bus it instead.
Shuttle buses will be replacing the trains between the Allandale Waterfront GO in Barrie and Union Station.
"Buses do not have the same capacity as trains and shuttle buses may experience longer than normal train times," Metrolinx stated.
Downsview Park GO will not be served by any GO bus replacements and customers are advised to use the TTC instead.
Regular GO train service will resume on Monday, November 6.