We've been taking Kai on trains and trams to expand his horizons. Trains and trams are "choo choo". Buses are "bai". I have no idea why buses are "bai". They just are and I've argued this with Kai until I'm blue in the face.
During his first trips he was quite silent, unsure what was going on around him. But the second time I took him to London Victoria station, he was already used to the experience and kept trying to get off the train every time he heard the doors open.