So - I followed the MQ and got to the Institute.

I kept telling them I didn't know if I wanted to join and never did - I simply met different department heads and then left.

I haven't done any quests for the Railroad - although I was welcomed to their headquarters after Freedom Road b/c I had killed a Courser in the MQ.

I went to the Railroad and Desmonda (sp?) told me it was too late and to leave?!?! This makes no sense - I went to the Institute but I didn't join!

The only thing I did was download a holotape b/c I used the minutemen to build the teleporter thing (I chose them b/c they seem neutral to everyone) but I didn't give the holotape to anyone - figuring I could choose?

So...... what the heck?

How am I supposed to know when I can/can't do faction quests if it doesn't follow logic? I haven't helped anyone yet (except minutement and I did the BoS first 2 quests but never went to airship) so I thought I'd get the lay of the land so to speak and then help.... after looking around the Institute I was convinced they were goons and went to Railroad but......

Help?

You had to have done something, or said to someone that canceled the ability to go through the railroad line.

after you meet the department heads, Father should say "will you join us?" or somesuch. If you said "no" and then the railroad wouldn't let you in I'd file that as a quest bug. I'm not sure what happens if you just TP out before Father gives you that choice.