True, the Guelph siding alone isn't enough to get counterpeak service past the 30-minute peak service, but combined with a passing opportunity at Georgetown it's enough for hourly off-peak service in both directions, given that Guelph and Georgetown are precisely 30 minutes apart.
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Georgetown station was supposed to be reconfigured this year, but I haven't heard anything on that recently. It's possible that those plans have been put on hold pending decisions on the future track configuration resulting from the planned GO-to-CN flyover between Georgetown and Mount Pleasant.
In the meantime, it seems like we could make a quick-and-dirty passing location at Georgetown simply by lengthening the south (VIA) platform to support GO trains. The existing track configuration would allow the eastbound and westbound GO trains to cross CN traffic simultaneously there, operating on the south track east of the station. This leaves the remaining 50-minutes per hour free for CN trains to pass through. The Kitchener line business case implies that CN has agreed to hourly GO service with this type of operation, even prior to the flyover being built.
West of Georgetown, the Metrolinx Guelph subdivision joins into the CN Halton Subdivision from the north - eastbound trains would cross over here to reach the south CN track. Westbound trains would stick to the GO track on the north side of the corridor.
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There seems to be enough space to lengthen the south platform to GO's standard length of 314 metres. Eastbound and westbound trains would meet here.
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East of Georgetown, GO would operate on the south track in both directions, avoiding any further conflicts with CN. The Metrolinx Weston subdivision splits off to the south at Bramalea.
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Given how quick it would be to lay down a temporary platform extension at Georgetown, I figure that hourly midday service could begin as soon as the second platform at Guelph station is complete.
As for counter-peak service, Metrolinx has also stated that they will build a passing track at Acton, which would then provide sidings every 15 minutes along the line. There is already a passing track 15 minutes west of Guelph, located just east of Kitchener Station. That would allow an hourly counterpeak service to meet the half-hourly peak service.