Why can't they run all day go service? Does anyone know what the cost of a go train is? Or is it an issue of track capacity?
Three reasons:
1. the project was never completely finished. The fourth track, which would provide all the necessary track capacity without any conflicts, was never laid, though it has been roughed in. (Forget, for a moment, that the Japanese can run two-way local and limited stop services on a single track corridor between Kyoto and Nara.) It will also require expanding the underpass at Highway 401 and removing Etobicoke North Station.
2. At Bramalea, the corridor joins the CN mainline. GO can run hourly two-directional service past there (it does during midday weekdays), but it needs a third track at least through Downtown Brampton, or the "missing link" bypass. Still, GO should be able to run more frequent local trains to Bramalea, once station work is done there, but until problem 1 is addresed, it's unlikely.
3. UP Express was underbuilt and designed as a premium airport service. The stations at Pearson and Union can only handle three-car trains, which are built for comfort, rather than capacity. as a political project, UP Express had priority over the larger GO market.
The "entitled" Weston and Bloor commuters were never really considered, until Metrolinx was embarrassed into making UP Express useful by lowering fares and making it useful for commuters.