Celebrim
Legend
For me it's pretty simple. If there are predetermined scenes, plotlines, story beats, encounters, or solutions to problems, that the PCs cannot change and must engage with the way the GM has decided they will, it's a railroad. Functionally, I don't see any difference between a "linear" game and a railroad, other than whether or not the players are okay with it. If the players are happy to jump through the GMs hoops exactly as the GM intended, then it's a "linear" game. If the players aren't happy about it, it's a railroad. I don't think there is anything wrong with running/playing a railroad game. I do think it completely fails to embrace the one and only thing that makes TTRPGs unique, which is the ability to wholly customize the experience for the people playing the game. Basically, in a railroad game you could swap out any player or PC for any other player or PC and not much about the main events would change. Whereas in a game where everything is tailored specifically to the particular players and PCs, then swapping anyone out would drastically change the game via the butterfly effect.
How many players are normally at your tables?

