EzekielRaiden
Follower of the Way
Of course, this is a bit like saying "It's not mind control if the person never tries to do anything the mind control would prevent."That's a really good point. If the DM never denies any of the players meaningful choices - no railroading occurs.
It's also a good way to look at why linear =/= to a railroad. If the players are voluntarily following a linear path (because that's all there is and they are aware of that, such as an adventure path) they are not being denied any meaningful choices.
And that's not even getting into the whole "the odds of no rail ever getting tested vanish to 0 rapidly with game length" thing. Unless you're working with people who are functionally carbon copies of you, people think differently from one another, even if only in small ways.