On the London train recently I was using a bluetooth keyboard with my iPad, and it was a very good match for the limited space in the seat. The iPad sat on the little table-shelf thing and the keyboard on my lap, partly under the shelf.
It may not look it, but it was really comfortable, and the Vodafone 3G connection held up well. There is no way I could have done productive work in this space on my laptop, but I managed to fire off quite a few emails and Skype messages on the iPad, and, of course, it had enough battery for the journey there and back and quite a lot of use in-between…