I actually did not bring this up for the sake of just passengers.... but I am going to be bringing up the Hagarsville Sub below.
You are speaking about the A-line. After Hamilton's East/West LRT (the "B-Line") the James-Upper James corridor is next on the list. But I personally don't think the case for that is super strong all the way to the airport yet.
....This is definitely not what would be repurposed, if anything ever were. We can muse about bringing back old rail lines, but the case for Radials is far harder to make. Like with most, the corridor is basically gone, and its too urbanized to be worth bringing back.
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No, I wanted to suggest bringing back part of the Hagarsville Sub for freight use. Passengers could be a side benefit- perhaps its good enough to supplant any A-line past say Rymal, but I digress.
The bread and butter of YHM is cargo, and the City is betting on this. Its a rare spot where greenfield land is plentiful. The only problem is proximity- its not near the industry at the lake, and it's about as far from the industrial heart of the GTHA (YYZ) as one can get while still within the region. There is only passable highway access, and while some plans might fix that, maybe we could do even better with rail instead.
Before I get into it, this thing would need to be about 50% grade-seperated. Not cheap, but I have a feeling some folks could clue me in if I'm going too far. This is way more than one bridge though.
So heres my idea... Run a line from the Grimsby Sub up the RHVP and rejoining the Hagarsville alignment near the Linc/RHVP bend. Run south; I don't know if air-rail direct intermodal is a thing, but that would determine the location and size of an airport spur, hence the splits shown. IIRC, the spur to Pearson for the UPX isfar more curvacious than the Kitchener Line, so it would be easier to only run small trains here too.
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My piece de la resistance is once again suggesting to bring back CN's beach track as a bridge (ideally forgetting about expanding the Skyway) but that's off-topic. If people are interested I can move this to the fantasy thread.
Anyhow, I figure the case is you can get some of that heavy industry/intermodal stuff out of the core, and grow the amount of industry while doing it- for the airport
and RRs. If we can get these guys on board, making the case for a passenger track (or two, grade permitting...) should be marginal.