PTC has no bearing on how many trains you can put on a set of tracks.
Exactly right, although I hope PTC is installed (for safety rather than capacity reasons).
A four-track route can easily handle both stopping and express service of very high frequency, with or without PTC, except....if one of the services branches mid route. The issue is, if trains switch tracks, they cross the paths of other trains. Run a Pearson-bound UPE train on the northmost track, and it will have to cross three other tracks when it reaches the Pearson Spur.
There is a minimum time that the train's route has to be lined in advance of its arrival. During that time, while the crossover movement is routed, none of the other affected tracks can be used for through movement. And there is a minimum stopping distance for these other trains. With the new signalling on that line, trains get their first restrictive indication several blocks away from the "home" signal where the crossovers are located. That restrictive signal buffer forces trains to slow down, PTC or no PTC. Any train needing to crossover may take several minutes' capacity out of every track it crosses.
Ever since UPE was promised for the Pan Am Games, its needs have taken priority and GO has been an afterthought on that line. RER and HSR spec are after-the-fact bolt-ons. The RER and HSR service specs are still so loose that it's impossible to say exactly how it should work. No wonder that the design may not be optimal, even after all the money spent on GTS.
Even with four tracks, UPE sucks up more than its share of capacity because crossovers are inevitable. There has been talk of building a flyover at Wice, although I have never seen this mentioned in any public document.
Same thing will arise at Scarboro Jct, by the way, when RER/ST is cranked up on the Unionville line. VIA, and through GO trains to Oshawa and Bowmanville, will have the same constraints.
- Paul
