Hm. I don't usually get involved in these kinds of discussions, because I don't quite have the best handle on my own optimum admixture of scripted plot versus aimless dithering. I do know that having attempted to run a few dungeon-heavy sandboxes in recent years, they've wound up consistently boring all of us to tears. So some higher-than-average proportion of plot, somewhere in the vicinity of 60-70% scripted railroading, is the minimum needed to keep my group and I entertained by the game. Your mileage may vary.
Don't forget, though: railways aren't linear tracks from point A to point B. They have switches, branches, turnarounds, and stations where you can make a changeover to another line. The choice must come down to two diametrically opposed DMing styles, the sandbox referee or the railroad narrator, I suppose I have to choose the latter.
The question is, are you the ordinary sort of railway conductor who lets passengers on and off at the stops they want? Or are you some kind of demon engineer, straight out of an old west ghost story, who traps the poor and unfortunate souls on the train and keeps them in a choice-free and meaningless limbo for eternity?