I've had the good fortune to use WiFi on the Heathrow Express when I'm in London. The experience, which is made possible by T-Mobile connects train mounted access points to 3G radios that are mounted in each car on the train. The experience has been good enough to hold video calls using SightSpeed and Skype without any difficulty, as well as to make VOIP calls using Truphone.
This makes this news report from WiFi Net News rather exciting, as the state of Massachusetts train authority is rolling out WiFi on all their trains for the princely sum of $300,000.
As gas prices remains high, more people will begin to look at transportation alternatives to get to where they're going. Based on experiences in London, WiFi on the train sure makes the ride go faster and more productive.
Update--Pal Joe Brancatelli of Portfolio 2.0's Seat 2B points out a service called LimoLiner that runs between New York City and Boston. It's a high end motor coach with WiFi....nice.