You probably know you can check the schedules and fares by looking at the SNCF site (or just Google SNCF). The trains work well (much better than the bus) if you can make the 7:54 train. They run several through trains per day and these only take about 2 hours each way (vs 3 hours on the bus). The cost for 2nd class is about 48 Euros (round trip). However, if you miss the direct routed trains than you must change in Rouen and the time of the journey increases to 3 hours. As to the reliability of the SNCF, well, it is a unionized operation which does well as long as the union wants it to go well. If the workers are unhappy (God forbid they would have to work more than 35 hours a week) than you will not be a happy camper!