They are trying to minimize scheduling conflicts and delays on all the lines by not having trains from other corridors cross their paths. It significantly diminishes the numbers of things that can spread delays through the network. Switches that don't switch don't fail. Trains that get delayed on the line don't disrupt the schedules from trains on other lines. The corridor is being set up so the north two tracks are Barrie line and only stop at a potential Spadina station. The middle four are Kitchener and UP tracks, and the south one (eventually two if Milton corridor gets frequent service) is the Milton corridor. Out in Scarborough they are creating a rail underpass onto the Stouffville line to avoid those conflicts too. Could they just put in switches... yes, but that would limit frequency and increase the chances of system delays. The goal is to build a network that is electric and could potentially operate like a subway line some day if ridership demanded and the coordination of 3 frequent trains lines, one frequent airport line, one lower frequency commuter line, two infrequent national rail lines, and frequent equipment repositioning is difficult enough. Also, the operations of the stations, wayfinding, and where passengers should stand not getting in the way of other lines is simpler when Spadina-Front station north track is Aurora bound, the south track is Union bound, the King-Liberty north platform both tracks are Bramalea bound, the south platform both tracks are Union bound.
