As far as I know, all the vehicles run on Main Street all day during the day, except for when they are parades on. I am not sure how late they run at night!
At the end of Main Street, in front the the Train Station are stops for the vehicles and it indicates on the sign for which vehicle the stop is for. At the other end of Main Street, in front of the castle, is another set of stops.
Just stand in front of the sign for the vehicle you want at either end of Main Street and you will be able to board the vehicle when it arrives at the stop.
If I recally correctly, the bell for the fire engine is at the back so you would have to sit there but I may not be remembering it right.
Our favorite is the Omnibus, on the top. The view is amazing up there!
This was something we never did until a few trips ago and now it is something I love to do.
The Main Street vehicles are an overlooked attraction, in my opinion.