Extra Capacity on Northeast Corridor

I cannot answer specifically how much additional capacity can be had on the NEC. However, with the recent funding of long-needed catenary, power supply and signal upgrades, I expect these improvements will ALLOW more capacity than is the current norm. In addition, when Portal Draw over the Hackensack River gets replaced, a real trouble spot bottleneck will have been eliminated, also increasing capacity.

Now if Amtrak's Gateway tunnels under the North River get built, even more capacity becomes available.

Not really an expert here, but as far as I know...

Capacity is constrained on the NEC primarily by choke points in the infrastructure which slow the trains down to a crawl - the ancient bridge crossings in New Jersey and Connecticut, the tunnels in Baltimore and NYC are the most frequently brought up. Penn Station is also at capacity from a platform standpoint, and pretty much most of the line would need longer platforms to extend the length of the trains. I think that the century-old yard between Penn Station and the Hudson tunnels is probably a bigger constraint that the tunnels themselves...

Amtrak in general is constrained by motive equipment - the Acela sets are pretty much in full use, the AEM7s are in process of being replaced, and the Amfleet cars are now over 30 years old.

There's off-hour capacity to be had, but during the commuting day, the system is darn close to capacity as-is.

Ironic, however, that if one were to go railfanning during rush hour - especially anywhere in PA, or more specifically in DE or MD, you could find yourself bored out of your mind waiting for something to photograph.  Amtrak runs 4 trains per hour in each direction at it's busiest, maybe more like 3 and off-hour, just 2.

Maybe there is a lesson to be taught from Japan which advertises trains every 12 minutes or so?  Or Europe where TGV's, the Eurostar, and even ICE trains mingle on the same line.

/Mitch