It means either 'went' or 'set off'.

поезд пошел - the train (has) set off. (The train has changed from a stationary state to one of motion)

Сегодня утром пошел с Борькой на корт. Там я играл с ним- This morning Borka and I went to the tennis court. There I played with him
(I think here пошел moves the action on to the tennis court. You are more interested in your having gone to the tennis court and what happened once you got there than what might have happened on the way. And сегодня утром means we're talking about a one-time action.)