Majoru Oakheart
Adventurer
If you're playing a linear, scripted adventure and try to get out, and the DM bludgeons you back in line, that's railroading. Until you try to get out, you can't tell if you'll be railroaded or not.
And the key here is that if it's done correctly, you might not even KNOW he's bludgeoning you back into line. There might be perfectly reasonable reasons you can't take the action you want to.
In my mind it is the difference between "The mayor says that he fears for the lives of the people who were kidnapped and he fears that if you don't leave now to save them, they will all be dead shortly. One of them is the only person alive who knows the secret location of the artifact that you need to destroy the BBEG" and "A wall of solid darkness forms around the town out of nowhere that blocks off all routes except the one to the cave where the creatures took the people they kidnapped."
The first seems like a reasonable thing to happen and gives us urgency to save the people. The second makes me question why the DM is so bad at DMing.
But I do think that a lot of it comes down to the players tolerance to "railroading". I've seen some players who are VERY independent and hyper-vigilant for "railroading". Even if the former situation listed above, they'd be the player saying "What? The one person who knows the location of the artifact we are looking for just HAPPENS to be one of the people kidnapped? And if we don't move quickly, they'll all die? You just don't want us going the next town over to visit our rich uncle like we wanted to, right? Well, screw that...I refuse to put up with your railroading."
That's when you start coming into conflict with your players in a linear-plot game. We don't have that problem because we don't have any players who insist on doing their own thing regardless of what is happening in the game. I've run into a couple of them, however. They almost always ruin the game for everyone and it ends with plot derailed so badly that it can't continue. But they are happy because they didn't want to play in a "railroad" game anyways.