Otto Vondrak wrote:The H&M tunnels were never designed to accommodate full-size passenger trains, only 50-foot rapid transit cars, so I don't see how this would be possible in any scenario.
Technically true for the tunnels that were started or completed by the H&M. The original 1874 design and construction was for full-size steam-powered freight. The work done after the project was taken over by the English engineers was an intermediate size.
If you look out the front of the train during the run from Christopher St. to New Jersey, you can see the tunnel getting larger in steps (particularly obvious if you're looking at the ceiling). By the time you see the illuminated "15th Street Shaft" sign on the left, it is full-size, but steps down to H&M size right after.
I don't think IRT dimensions and H&M dimensions are compatible, so I'm not sure that the IRT could send their trains into the H&M tubes. Also, let's not forget, when H&M was under construction, IRT was a competing, for-profit company. I don't see much chance of cooperation between McAdoo and Belmont.
The dimensions are similar. There would probably be some clearance issues fitting IRT cars into the PATH tunnels, though the reverse should work (H&M flat-sided cars, not PA-series cars). There were similar clearance issues when the PA-1 cars came to the property - pieces of the side bench needed to be chipped out in spots, and the unidirectional crossover west of 9th Street was taken out as the PA-series cars couldn't clear it. Interestingly, not all of the clearance issues have been resolved, even to this day. If a train of PA-series cars operating in single-track mode comes into Grove St. from the east, on the south (normally eastbound) track and crosses over to the normal track west of the station, operating at the maximum speed permitted by the signalling, it stands a pretty good chance of scraping the center wall. I've been in a car when that happened - it was quite surprising.
A bigger issue is that the trip arms are on opposite sides on the IRT and H&M/PATH. That means that unless the lead cars (which means both ends, as there were no loops in the midtown service) were fitted with dual trip arms (as some NYC Subway work cars are), you could get a car-length into a protected block before being tripped.
It should be possible to create a special "joint service" car, possibly fiddling with the distance from the trucks to the end of the car, and installing dual trips.
And the H&M / IRT got along better than you would think - each leased space to the other at Christopher Street (IRT 9th Avenue Elevated), and the un-completed H&M Astor Place spur would have come out in an IRT station (though not sharing tracks).
* Semi-related clearance trivia - the tracks around the Morton St. curves (particularly eastbound) are exceptionally low in the invert. False 3rd rail was installed to keep the shoes from flapping against the side of the tunnel and arcing. You can tell the false 3rd rail because there's no cover (it is actually wood boards that are very shiny from rubbing against the car shoes). Correcting this by adding more gravel to raise the rails was considered in the early 1960's, but rejected as service would need to be suspended (or at least curtailed to overnight levels) for an extended period while the work was being done. This is what caused the bowed-out sides of the PA-series cars to have clearance issues in that part of the system - the bowed-out part was supposed to be above the height of the side bench, but this lowered it to the point there were clearance problems. You can see a lot of spots there where the top edge of the bench was chipped away to maintain clearance.
* Semi-related Astor Place spur trivia - the original ring segment erecting machine is at the end of the eastbound Astor Place spur. You can't see it because there's a lot of power / signal equipment cases blocking the view, the rest of the tunnel isn't lit, and it is somewhat far down the tunnel.