I thought the train ride was just "okay". I nice, relaxing ride, but if you don't have time for it, you won't miss much. I'd guess it's about 20 minutes long.
We boarded at Main Street, and from there went through Adventureland, where you see nothing but.....trees and foliage. At Frontierland you get a glimpse of BMRR (I think), and also the Native American village. They also have a stop at Frontierland. Then on to the next stop, which is in Toontown. After that, back to Main Street. Not much to see on the way there either.
If you're looking for a nice, scenic ride....we much preferred the steamboat ride in Liberty Square. It's also about 20 minutes long, with a bit more to look at.
Kelly