The fact that someone agrees to play a railroad doesn't make it not a railroad.Your definition of railroad is far, far broader than most people's definition. A campaign can be linear and give PCs autonomy on approach while still having a clear default outline. If I'm playing Curse of Strahd, I may have multiple ways of approaching the campaign but when I agreed to play the game I knew what I was signing up for and that there would be certain aspects of the game predefined.
And linearity seems to be a defining characteristic of a railroad, rather than some sort of contrasting notion.
Useful for whom?saying that a DM's campaign is a railroad is one of the most negative things you can say about a DM and rightfully so. But defining railroad too broadly like you do and it becomes you stating a one-true-way to run games, not a useful descriptor of a style of game DMing to avoid.
It's useful for me to be able to characterise an approach that I don't generally enjoy either as player or as GM.
I mean, you're hardly shy about forthrightly stating your own preferences and criticising those of others!