The topic of using existing commuter rail lines in GTA has been brought into discussion a few times. With politicians bringing ideas to solve transit woes in the GTA and in Toronto by creating a Downtown Relief Line or One City Transit Plan, all of which that incorporate existing rail lines. They are on to something.
Adding specialized TTC Trains or by having special Go Trains running frequently at peak times could help solve some of the transit woes face by downtown Toronto and the GTA. Instead of investing cost and labour into new infrastructure it would be feasible to work with the existing infrastructure to improve it.
At the moment most trains on the GO Line operate in the early morning and in the afternoon, brining workers into and out of the city. With a few lines offering all-day service. What should be done, is turning all lines into all-day service. This would eventually lead towards having a rapid-transit system on the rail line.
More importantly, having a rapid transit system on the rail, commuters would be able to choose between taking the TTC or the GO. This would hopefully reduce some of the stress on the TTC subway lines.
This idea is not far off and could actually be done with the correct investment and planning. I suggest you take a look at this article Make the effort, and commuter rail can be as effective as rapid transit ,which provides more detail on this topic.
Read More: The case for a Regional Transit Line
Image by: Nathan Colquhoun