Can you railroad a willing player? (Forked from "Is World Building Necessary?")

[pawsplay]

What do you mean by that? If I set out by Ferrari on the road to Milano, and end up by GM Fiat in Roma instead ...

That's a rather more literal interpretation than what I intended when I wrote that in quotation marks, but so be it. There are games where this is a perfectly acceptable occurence, something strange that the PCs must now respond to. If, on the other hand, you are playing a relatively realistic modern spies game, this becomes quite strange; extraordinary actions by NPCs might still steer them, but you can't really stop them from going "off the rails" except by extreme measures. If they insist on going to Milano, the game will either be about Milano or the ridiculous obstacles they faced trying to get there.