Disclaimer: I think railroading is distasteful. My definition portrays it as negative.
Game master’s decisions are railroading when
- Players assume they can have an effect on a particular aspect of the game.
- There is no credible diegetic (fictional, in-game) reason for players to not be able to affect that aspect.
- GM’s decision negates or marginalises player input with regards to that aspect.
I think the first part makes railroading inherently bad and feel railroady to the players.
- A band of merry PCs must decide to go either south or west because they are at crossroads. Wherever they go, they shall encounter the same group of bandits. They have no reason to assume that there would or would not be bandits at either direction. Not railroading.
- As above, but reliable sources tell that there is only one group of bandits and they prey on the south road. The group of PCs take pains to make it seem like they are going south, but actually go west. They spread rumours, check out that they are not bieng spied, etc. Still they face the bandits. This I would call a minor instance of railroading, unless the bandits had a good reason to go west and it is revealed to the players.