“Railroad” is a dirty word for many people who play roleplaying games.
From a wide-angle view its use probably just means that the person running the game and the player making the complaint haven’t been on the same page about what makes the game fun. It could be everything from the players rebelling against their controlling frustrated-novelist DM to a single player who balks at the fact that he can’t bail on the adventure that everyone sat down to play, and it wouldn’t have gotten to that point if everyone was on the same page.
A clash of expectations, then, and something that in many cases can be avoided with communication between or experience with the members of a group.
Specifically, though? It refers to the player experience of discovering that the world their character inhabits has hard, sometimes arbitrary-seeming, boundaries that they’re unable to overcome.
“The sword-fighter knows that there is no way he can win this fight.”
“You find yourself chained to the oar of a slave galley.”
“This prison has been your home for three months.”
“You can’t leave the city, the ways south are blocked.”
I can’t tell you how to get rid of railroading in your games, because what one group considers a wide open adventure might look like a straightjacket to the group that often ends up building strong relationships with random – previously unnamed – NPCs.
But I can share one trick that helps mitigate what I think is a major source of the “Railroad” accusation.