Hussar
Legend
While facetious, I think EW does hit it on the head nicely.
Starting out in medius res is, I suppose, technically railroading in that you have taken away choices from the players, but, it's pretty acceptable since it's just the starting point. Most adventures actually have some sort of starting point - whether it's the entrance to the cave, undertaking that journey, or whatever. The initial set-up for many adventures is pretty fixed. If that's true, does it really matter that the DM let the players make choices which led to that initial starting point, or simply starts them there?
If you're playing White Plume Mountain, does it really matter, in the long run, if you play out the overland journey? Is it railroading to skip to the entrance? IMO, maybe, but, again, it's pretty acceptable.
Where railroading become unacceptable is when the DM limits reasonable options to force you to a specific end point. Starting you in Hommlet isn't railroading. Forcing you to talk to specific NPC's and engage in the setting in a certain way, is.
