To be fair, the 56-minute number includes a transfer at Aberfoyle. If they ran a direct bus route it would probably be scheduled to take about 25-35 minutes.
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The problem with a bus is of course that it would be a lot less predictable than the train due to potential traffic, which would require much longer scheduled transfer times to trains in Guelph to avoid missed connections. GO typically schedules buses to arrive 15 minutes before the train, but if the Cambridge-Guelph service is a train it could be scheduled with connections as tight as 5 minutes in Guelph.