It's not looking good today, in any case:
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I had also noticed the 10 mph operation west of Kitchener station. I think your hypothesis is correct: new slow orders west of London have decimated the on-time performance of the London GO train and VIA 84 (Sarnia-Toronto).
Via 87 (Toronto-Sarnia) was hopeless regardless, but at the very least they should shift its departure time from Kitchener about 8 minutes earlier to account for the Metrolinx track upgrades between Georgetown and Kitchener, and give a bit more time to tiptoe from Kitchener to London.
Also, it is occuring to me that the London train departs for a more practical time for me (11:33 Central European Time) than the for the people in London Ontario.
I'm wondering if we could make a program to automatically estimate the current track speed profile summarize using the GO and VIA GPS traces. If you plotted speed over distance for all trains in a month, then traced the maximum speed for any train for each given segment of track, you would have a "maximum speed profile" for the line which could be compared from one month to the next to identify new or lifted slow zones.
I personally am not tech savvy enough to do it, but it seems like it would be quite doable for someone who has worked with GTFS data before.