No, the constraint is the fact that all except a few trunk freight lines are designed for relatively infrequent and long freight trains, with a single line with "passing loops" (this may be an aussie phrase) or sidings that the train going one way sits on until the train going the other way has passed.

Since the freight has priority, once a passenger train in the US leaves the passenger only lines in the Northeast, small delays cascade into massive delays as it sits in a passing loop, waiting for train A to go past a passing loop further up the route so that train B can get off the passing loop and go by the Amtrak so that the Amtrak can move. A delay of 15 minutes can easily spill over into a delay of three to five hours in tack access delays.

Now, most of these are in rail corridors that once had two way track (because there used to be far higher frequency of passenger trains and much more local freight travelling by rail). So in these corridors, you can take advantage of the existing rail right of way to  put in a dedicated passenger service track.

If the frequency is brought up to the level where passing loops are needed at all, the passenger line can take advantage of the long stretches between freight trains by having switches that allow either the existing freight track or existing freight passing loop to be used as a passing loop for the passenger services.


I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.