These teenaged boys will grow up and they will be exposed to things worse than what they see on the bus.
Instead of making the bus or train taboo and not allowing them to go on it, it might be a better idea to teach your children self control.
Teach them that it is impolite to stare and teach them that sometimes they will come across things that are not appropriate to look at and when that happens they should remember that they are above that.
Teaching them to take charge of their actions will help them more in life than teaching them that everything is bad and that normal everyday things should be avoided.
You won’t be able to shelter them their whole life, and by not allowing them to go on the bus because they might see soemthing they shouldn’t will just cripple them.