Just to get into fine detail about why Scarborough needs attention....
Through this location there will have to be a local track for eastbounds, and a local track for westbounds. At Scarborough, there is a branching with two tracks to Stouffville and two tracks for Oshawa. Stouffville trains will run every 15 minutes, and Oshawa trains will run every 15 minutes. Coming out of Union, and going towards Union, that's a train every 7.5 minutes each way. Over time, those headways may be reduced.
The problem is that no matter how you use the trackage, with a 'flat' junction there is a need for one set of Stouffville trains to cross over one set of tracks used by Oshawa trains. And vice versa.
Only happens once every 7.5 minutes....Sounds simple? The devil is in the need for the route to be clear a couple minutes ahead of each train. If the route isn't clear, the approaching signals will be restrictive, and trains will have to slow down. That reduction will begin several signal blocks - ie several miles - away from the crossover point. So that once-every-7.5-minutes use of the crossing point requires up to 4-5 minutes of dedicated advance signal time at the crossover. During that time, the crossing is not available to trains on the other route. Even with precise scheduling and operation, that leaves little room for train delays.
There will also be two express tracks for use by VIA and express GO trains. In theory, the two southmost tracks could be used as express pathways, so they don't have to factor in the math.....but....if the middle tracks are used as the express tracks, as the present platform placement along the LSE dictates, the crossover movements not only have to clear the opposing local GO movements, but the express tracks as well. In a flat layout, they would be blocked once every 15 minutes for 4-5 minutes.
The solution is likely a grade separated flyover or flyunder similar to Rahway NJ. There are plenty of precedents, it's a simple matter of figuring out how to rough it in. The grades and road crossings involved are what may make it desirable to move the platform locations for GO Scarborough.
- Paul