Here's the explination from Reddit:
And if you notice in the wintertime, all SRT vehicles run on manual-mode (the two red lights on top of the carriages flash, and a blue marker light on the side is on, which shows a fault in the on-board computer). At least during the winter, the drivers are actually driving the train and relying on spacing the trains safely based on Transit Control. All other times especially during the summer, the trains are running in automatic mode. I assume this is because the contacts on the rails are blocked by snow and ice. But, the SRT trains especially fully-loaded cannot always negotiate the incline at Kennedy Station, so drivers have to put the train in manual mode to gain speed without the train slipping the tracks. It's fun to be in an SRT train bound to Kennedy going back and forth trying to negotiate that incline as it struggles to climb.
The trains communicate with a computer at Kennedy Station and at Transit Control in Hillcrest. The SRT is Toronto's only rapid transit line capable of Automatic Train Control, until Line 1 is upgraded.
Fun fact: the TTC's signature door chimes were introduced in the SRT