My husband and I will be taking the train from Arles to Avignon on Oct. 7th. Do we need a reservation?
They are local trains with no reservation needed or possible. Just buy a ticket and validate before boarding.
Please notice that you can book either the TGV service or the regional TER service. The "train a grande vitesse" is not so speedy in this case, since it arrives on the edge of town, followed by a shuttle bus ride. The TER arrives at the old central station beside the city walls and the train company estimats the time involved to be less than half the TGV itinerary. Mind you, the old train won't be as pretty or as comfortable.
Aesthetics are up to you, but I don't think the train between Arles and Avignon by "classic" line will be any less comfortable than a TGV.
My experience was no reservation necessary-I just took the local train service from Arles train station to the main Avignon station. This was a few years ago and there was no TGV service then between Arles and Avignon at that time. And I wouldn't take it even if available - the local trains don't take too long to get to Avignon and the main station is a short walk to things like the Papal palace. Also reasonably priced.