A video by the Joint Transit Association talks at length about through-running in New York – which lines are easier and which are harder, what some of the tradeoffs are, what sequencing works best with ongoing infrastructure plans starting with the Gateway tunnel. It’s a good video and I recommend watching – and not just because it gets a lot of its ideas from ETA reports but also because of its own analysis and own points (about, for example, Mott Haven Junction) – but it has one miss that I’d like to highlight: it neglects Penn Station Access West, the proposal to connect the Hudson Line to Penn Station via the Empire Connection.

The issue is that without the realignment, too many trains would be going into Grand Central – all preexisting Metro-North service minus trains diverted to Penn Station Access. We expect all this through-running infrastructure to add to peak demand substantially. Today it fills about 50 peak trains per hour, which a four-track trunk line would struggle with (Metro-North runs trains three-and-one at the peak). Even with diversion of 6-10 trains to Penn Station Access, the extra demand would saturate the line. Penn Station Access West is important in reducing this capacity crunch.

The realignment is both important and cheap. The Empire Connection exists and the tunnel has room for two tracks; it needs a short realignment to reach the right part of Penn Station – the high-numbered northern tracks as in the image, where today there is a single-track link from the Connection proper to the low-numbered tracks – but that realignment is much cheaper than a full through-tunnel such as between Penn Station and Grand Central or the various lines to Lower Manhattan mooted for longer-term plans.

The total capacity produced should be every train that doesn’t have to go to Grand Central. It’s hard to exactly say what the split should be – there should be a minimum of a train every 10 minutes to each destination, if only to serve the inner stations that are (or would be infill) on the lower Hudson Line or the Empire Connection before the two routes meet at Spuyten Duyvil. Beyond that it’s a matter of measuring demand and seeing what the limit of timed connections are; ideally there should be 12 peak trains per hour on Penn Station Access West and only 6 on the preexisting route, up from 14 total on the Hudson Line today due to service improvements brought by through-running and related upgrades. This is necessary to create the capacity to run more service on the other lines – today the Harlem Line peaks at 16 trains per hour and the New Haven Line at 20, but these upgrades would create a lot more demand and my assumption in sketching through-running tunnels is that the Harlem Line would need 24 and the New Haven Line would need 18 to Grand Central and 6 on Penn Station Access.