I have not used the train, but I often watch from there, and the trains are coming and going all day long. It does seem a very popular service, and of course, you are not stuck if the weather turns bad.
I was at Bungalow by the tram last year at the TT. Had to wait about 15 minutes to get on board. Please note that you have to buy your ticket in a small stand close to the trams before you get on board. Return trip from the mountain, no problems but I left early before the last riders had passed for the last lap.
Don't be that optimistic! The light in the tunnel can be a train.