we've noticed some interesting segregation on the trains and busses here in vietnam.
on busses, the tourists are always at the back of the bus. most tourist busses will also take locals on them when they're not full, and i've seen the people running the busses ask tourists to move back in the bus so that locals can sit up front. kinda funny.
it also happens on trains, but for a compleatly different reason. most tourists book trains through their hotels. all the hotels but their tickets through the same company in each town that goes to the train station and buys up entire cars. so normally when you book your ticket in your hotel, your pretty much guaranteed to get a seat next to other tourists. your other option is booking at an official train office. we did this once and we got seated with some locals. but this is a bit difficult because getting to the booking offices is a pain, and there may not be anyone that speaks english there. also the trains fill up earlier, and when the official office tells you there is no space, there may actually still be a space that you could get through the hotel, since they have essentially reserved a bunch of tickets by buying in bulk.