The AMT Montreal-Dorion route, which stops at Dorval, runs service throughout the day including weekends. Service is nowhere near as frequent as once an hour however, except during the weekday rush.
The Monday-Friday westbound schedule is here:
http://www.amt.qc.ca/tc/train/horaires/ ... =1&ligne=2
and eastbound is here:
http://www.amt.qc.ca/tc/train/horaires/ ... =1&ligne=2
Saturday and Sunday/holiday train schedules are also available on
the AMT website which is in French only (with minor exceptions) at this time.
You might consider using the
bus & subway (métro) to get around for part of your travels. STM buses serve most AMT train stations out to Sainte-Anne-de-Bellevue.