This was a surprise to me. It just goes to show that having a plotted adventure doesn't have to mean taking away player agency, as long as the GM can improvise good, in-character incentives to stay on track.Ding.
The above quote is taken from this post. I'm honestly surprised this point doesn't come up more often. Sandbox is not the only answer to the player agency dillemma; I think the poor reputation so-called "railroad" adventures suffer from is due mostly to, simply, poor GMing.
I'm currently running a Savage Worlds plot point campaign ("The Flood" for Deadlands: Reloaded) and have had three incidents in nine sessions where the players partially or completely derailed the plot. (Granted, this seems to be particularly endemic to Savage Worlds, which gives the PCs a fair amount of narrative control.) So what happens then? Improvisation. Creative problem-solving. The stuff that makes running games fun. Those "Crap! Now what?" moments are both totally scary and totally invigorating. And after the session, thinking about what can be done to bring the plot thread back into an intersection with the PCs, not by pallete-shifting, but by playing to their motivations and goals. Of course it also helps to write in extra sources of plot hooks or other information if the material you're working off of bottlenecks anywhere (something The Flood does a little too often, I'm afraid).
At any rate, as I said at the top: it was refreshing to encounter this attitude, that a plot-driven adventure need not be an agency-robbing railroad. Just trust the players to give you everything you need and the rest is gravy.