"Railroading is a pejorative term for a game in which something is accomplished."
Kenneth Hite, Dragonmeet 2010
I suppose it is these days. And yet I still use it. And I use it to mean this:
The GM pushing the players and their characters through a plot that they would otherwise leave.
By this meaning, a plot can be linear of "on rails" without being railroaded, since as SteveD puts it, "Nobody minds the railroad if the view from the carriages is pretty and the destination is Awesome Town." It's a question of whether the GM is dragging you along.
I've been guilty of this... and equally guilty of being entirely reactive and leading to the common joke around our table where the PCs' action is "we wait for external stimuli".