So Google maps says its 67 mins drive time from downtown Toronto to Pontypool. Thats not that long though I guess you should factor in the time of day you will be driving etc. Anything outside of normal rush hours you should be ok going east bound and then ok going west bound.
Any time savings you might make taking the train to Oshawa will then be eaten up by having to get the rental agency either at the end of your trip to drop off the car or picking it up.
Pontypool is a pretty easy drive from TO. Just take the 401 eastbound to the 35/115 Hy and then north to Hy 35 etc. Very straight forward and the cost of a car rental for the day will probably be the same as trying to take the Go trains. Also, having a car at your disposal for the whole day will mean you wont have to time your trip to get back to Oshawa to catch a train back into the city.
I would opt for a car rental for a whole day from Toronto. Any time savings would be negligible by taking the train.Edited: 3 years ago
Lucywestie is absolutely correct - you won't save any time by taking the GO train. You will however avoid having to drive in the city and/or on the DVP & 401 out of the city. If the latter is a concern for you, if you are unaccustomed to or uncomfortable driving in heavy traffic, then the GO/rental car option would work. As noted, the time of day you are driving should be taken into consideration and the weather conditions (you don't say when you are coming). But as he notes, it's a pretty straight-forward drive ... I do part of that regularly and although it can be frustrating at certain times of the day, it's very easy.
Unless you hit afternoon traffic, and potentially even if you do, it'll still be faster and easier to just drive.