The cables were severed late Tuesday night. The incident is being investigated as vandalism, according to GO Transit's service updates.
Crews attempted to repair the cables overnight, but signal testing continued Wednesday, forcing trains to travel at reduced speeds between Guildwood and Pickering stations.
Commuters can expect delays between 10 and 15 minutes.
GO trains operating between Oshawa and Union station have been delayed between 10 and 38 minutes so far this morning.