Has an airplane ever been launched from a train?
It's not a train, but Brendan O'Brien (with the O'Brien Flying Circus) has an act where he lands a Piper Cub on a moving trailer, towed by a car. There are some YouTube-clips showing this, including a take-off (around 1:50). Regarding your earlier comment on 'as close at it gets': I think this is it.