A lot depends on the kid and parents. Ours got a basic Lionel set when he was three, but we were willing to supervise. If it's someone you don't know, or whose parents you know won't be comfortable with a plug-in, a pack and play Thomas would do nicely (I have one ready for the toy drive, but with a nice storm coming it may have to wait.)
brio and Thomas wood push trains were really popular around here, too. when other kids came around, those were safe for all to play with and were sturdy enough for the careless or mean ones.
