Caltrain's electrification is
currently scheduled to last 8 years between awarding the contract and running the first train in service. That's for 49 km of double-tracked lines (around 120 track-km mainline plus maintenance/storage facilities).
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GO's electrification is for 260 km of lines of varying widths (
687 track-km), and electrifying Union Station will be considerably more complex than electrifying San Francisco's King Street station, which doesn't even have a trainshed. The new south platforms are being designed with overhead wires from day one, but at least some of the historical plaforms also need to be electrified before a single electric service can start running.
As depressing as it sounds, I wouldn't be surprised if 2031 is indeed the start of electrified revenue service on the central portions of the core network (e.g. to Oakville, Bramalea/Pearson, Aurora, Unionville, Oshawa) with the remainder of the core network (to Burlington, Barrie, Lincolnville) coming online incrementally afterward. From a railfan perspective, the bright side is that electric trains will start testing much earlier than that. My guess is that they will start by electrifying between Pickering to Oshawa as a testing segment since the Whitby yard is already designed for electric trains.