"They are going by bus."
Translation:Вони їдуть на автобусі.
I have a hunch that автобусом is more often used in the west, and на автобусі in the east.
Hmm.... I've never heard "в автобусі" being used as "taking a bus" (only meaning literally, "I'm on a bus right now" - Я в автобусі)