Just wondering... I know when you get to Molecular Level you have to choose a faction to build the transmitter, but up until then can you do the quest-lines for all three factions?

I don't really want to finish the main quest yet, but the quest-line for Railroad seems to have dried up apart for the repeatables.

You're not locked into the faction that helps you in that quest, you can still do the other questlines that open up.