I did the quest to meet the Railroad, the first one of the Brotherhood (help to get the deeep transmiter) and several for the minutemen (just recovered the castle).

Now, both the Railroad and the Brotherhood want me to do this one quest for them and after I will invited to join. So my question is, can I keep going and join more than one faction for a while or Im already forced to choose one side this early?

I really dont know much about each faction, so I would like to be able to join more than one, at least in the begining, and just after kwoning what they are about choose the one I want to end the game with it.