What's the nearest train station to Mont Saint-Michel? Do I need to take a bus to visit it?
How much time does it take? Can I make it in half a day?
Stealing shamelessly from mouviciel's comment (and memory), the correct answer is Pontorson, which is about 9 km from the Mount -- a bit of a hike on foot, but quite doable if you rent a bike.
Also, the article quoted in Nikita's answer is shameless rip off from Wikivoyage, which is particularly annoying since I wrote the original of that text. Time for a nastygram! (Edit: to the person who copied it, not you Nikita!)
Looks like it's Rennes.
If you book your train in advance, the ticket might be a bit cheaper.