B.H.
No carseat needed on any public form of transportation including:
bus
train
plane
taxi
limo
I've used one when I could but many many times I've skipped it or there wasn't a way to use one. The only time I've ever regretted not having a carseat is in a taxi in New York City wow they drive crazy but I've gone to the fair on a bus with my daughter plenty of times and it's a easy going drive.