Quick order-of-magnitude benefits of this new raised speed limit for the 1 mile (1.6 km) zone through Guelph:
1 mile @ 10 mph = 6 minutes
1 mile @ 45 mph = 1.3 minutes
Net time savings: 4.7 minutes
The trains of course need to accelerate from 10 mph to 45 mph within Guelph, but that should approximately cancel out with the time they used to need to accelerate from 10 mph to 45 mph west of Guelph (en route to 70 mph).
Kitchener Station - Guelph Station travel times assuming a conservative 3-minute improvement:
Current: 21 minutes (63.6 km/h average)
December: 18 minutes (74.2 km/h average)
By comparison, Google Maps estimates the car driving time at 29 minutes without traffic.
Once Kitchener Central Station opens (providing easy access to GO trains from across Waterloo Region) and Guelph Station gets its second platform (allowing hourly all-day train service), I expect that GO will capture a sizeable chunk of the Kitchener-Guelph travel market.
A 3-minute improvement would reduce the Kitchener to Toronto travel time to 1h38 for express trains (63 km/h average) and 1h46 for local trains (58 km/h average). We've come a long way from the 2+ hour travel times when the service was first introduced.
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On a semi-related note, I happened across this description of speed limits on the Guelph Subdivision (London-Georgetown) in a
TSB report on a derailment in 2004:
So it sounds like welding the rails would indeed be a prerequisite to getting the Kitchener-London speeds back up to the historical 70 mph (112 km/h), up from the current 30 mph (50 km/h) and 40 mph (64 km/h) speed restrictions.