Sorry if this seems like a repost. This is my map concept of the GO RER, which I have posted before. I just edited some very glaring errors from the previous one.
Does mean that line C (Richmond Hill) will have 3 times as many trains as on the west, since the west ones would have to branch 3 ways. How about pairing Richmond hill with the King City and wood bridge line. Then pair Markham With Brampton and Pearson. And make the Streetsville line the orphan with no pair on the east.