PS: If you want to imagine how the Guelph Sub is likely to be enlarged, have a look at how CN transformed its Halton Sub (former Milton Sub) from branch line to main line 60 years ago. They added sections of double track where it was cheapest to do so, but left the more expensive bits (particularly bridges) as single track. The result is a chain of passing tracks with 3-5 mile segments of single track connecting them. That layout has served CN admirably, and while there have been rumours of full doubletracking being in the works at various points, the further capital investment has never been seen to be worth it.
I would expect that GO would use a similar strategy. The impetus for total double track will only come if there is a push to higher speed, very frequent, mixed express-local service such as a HFR or HSR. Even at 30 minute 2-way headways, vanilla RER can live with segments of single track over the pinch points.
- Paul