For context, here's the current extent of double track on the Grimsby sub.
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It's already mostly double-tracked, the most notable single-track segment is the 16 km between Grimsby and St Catharines. The single-track segment in Niagara Falls probably doesn't really matter since that's after CN's trains split off, and hourly GO trains would presumably lay over in NF for about about 20 minutes which still leaves enough time to get to/from the existing double track in St Catharines. A second platform at NF station (high-level platform for US customs clearance perhaps?) would be enough to enable hourly GO service in addition to Via/Amtrak thru trains.
But as others noted, I'd be reluctant to commit public funds to upgrade a line still owned by CN. Both Via and GO have been burned by that in the past.
The bridge is on a dual lock along the canal that only has a capacity of about 1 boat per hour per direction regardless of the railway bridge. If hourly GO trains are timed to cross the bridge simultaneously in both directions it would have a negligible effect on seaway capacity, though it could potentially cause up to an hour of delay to ships if they happen to arrive at the wrong time.
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