Almost dead south of Abernathy Farm, north of dam, there's a hill on top of which there's random encounters around the campfire; my 1st time there was the encounter in which a synth confided to 2 companions being a synth. I tried helping it and I think I barely managed to stop the execution. When I heard of the Railroad, I immediatly remembered about it and wanted to help again.
I didn't go in Concord right away, I scouted the surroundings to try to spot possible traps (going in Concord at the same time); so that encounter was one of my 1st.