The problem is that these lines are still used for freight trains, and unless they're expanded to three tracks for the entire length, they will need to share the tracks. It's not unprecedented for transit and freight service to use the same tracks: New Jersey Transit's River Line between Camden and Trenton shares tracks with Conrail freight. However, New Jersey Transit has an agreement with Conrail that the passenger trains will only run from 6AM to 10PM, except Saturday nights. That might work in South Jersey, but it would be difficult in New York City, even in Maspeth.
But let's suppose that it was worth running passenger service just for the daytime and evening use. The River Line is a secondary freight route, and only runs a few trains a night, and the Bay Ridge Branch is even more lightly used. Good, right? Wrong.
There is a proposal for a Cross-Harbor Rail Freight Tunnel to connect the Bay Ridge Branch with the Greenville Yard in Jersey City. Currently, rail freight going from New Jersey to or through New York City needs to either be floated across the river on barges, or hauled over 150 miles north to the bridge at Selkirk and 150 miles back south. In practice, it's cheaper and easier to transfer the freight to trucks and drive it over the bridges. Of course that wears down and clogs up the city's bridges, highways and streets, pollutes the air and increases traffic carnage, hence the need for the tunnel.
According to the Draft Environmental Impact Study for the tunnel, insted of eight train trips a day, there would be 24-64 freight train trips using the Bay Ridge Branch per day (page 8-69 of this PDF), depending on whether one or two tunnels were built. The DEIS is very unclear on the anticipated number of train trips on the Hell Gate Bridge, but the total number anticipated for LIRR destinations (i.e. all other destinations) is 13-15 per day. It sounds like that means we'd see 11-49 trains going over the Hell Gate Bridge, but I can't figure out whether the prediction is closer to 11 or to 49. I'm going to assume the most extreme scenario of 49 trains a day; it's still less than four an hour.
The TriboroRX proposal also doesn't take into account the plans to run Metro-North trains from New Haven to Penn Station via the Hell Gate Bridge. This would run five trains an hour in each direction during peak hours, a substantial addition to Amtrak's current ten trains per day.
Adding it all up, it looks like the following by the hour:
|Segment||Max tracks||Current freight||Cross-Harbor freight||Amtrak||Commuter rail||total|
|Bay Ridge FP to Brighton Line||4||<1||5||0||0||5|
|Bay Ridge Brighton Line to McDonald||2||<1||5||0||0||5|
|Bay Ridge McDonald to BAT||8||<1||5||0||0||5|
Here's the bottom line:
- Hell Gate: Amtrak, Metro-North and freight trains could fit two tracks if the freight trains are all run on off-peak hours, leaving two tracks for TriboroRX service. Some Metro-North trains could potentially share tracks with the TriboroRX trains to make room for more freight or Amtrak trains.
- NY Connecting Railroad: Requires widening, time-sharing or commuter rail.
- Bay Ridge Branch from Fresh Pond to Brighton Line: Freight trains can run in one or two tracks, leaving two tracks for passenger service.
- Bay Ridge Branch from Brighton Line to McDonald Avenue: Requires widening, time-sharing or commuter rail.
- Bay Ridge Branch from McDonald Avenue to the Brooklyn Army Terminal: Wide enough for anything.