All GO trains will close doors to passengers one minute before their scheduled departure time starting Monday, according to Metrolinx.
The new move will help trains leave the station and arrive to their next stop on time, a statement from the provincial transit agency says.
"We have been communicating this to customers starting Jan. 2," the statement reads.
"GO will be closely monitoring customer feedback."
Metrolinx says it's been alerting customers through its public address system, digital signs in GO stations, GO Transit's On the GO alerts and onboard train announcements.
On Twitter, GO Transit said platforms without accessibility ramps will close first, followed by the accessible platforms.
The change will not impact UP Express or GO bus service, Metrolinx says.