Even in those approaches to GMing which make impartiality a virtue, it's impartiality
at the point of adjudication. Inviting people to play <this thing> with you because you think it will be fun isn't adjudication. Similarly, telling people "Hey, this thing over here will probably be fun" isn't railroading. It's just being up front.
Telling the players, at the start of the first session of a classic D&D game "You're at the entrance to the dungeon", isn't "pushing the players in a direction that they did not choose". Presumably the players turned up to play a game of classic D&D. And that's how the game works!
Likewise, when I write up my Burning Wheel PC Thurgon - a knight of a religious order, whose backstory includes that he has been dispatched on a mission by the Knight Commander of his order - the GM is not "pushing me in a direction I did not choose" by starting things with Thurgon patrolling the border between Ulek and the Pomarj. That is just starting the game.
Classic D&D isn't a railroad, though. Dungeon-crawling, of the classic style, is a type of semi-freeform wargame. It has the basic structure that I described above: the players, who start somewhat blind, make "small" moves to obtain information about the situation, and then - having obtained that information - make "big" moves to obtain treasure, and thus progress in the game.
The GM doens't need to pretend that raiding the dungeon is just one option among many - raiding the dungeon is the game! What the GM does have to do is (i) present a situation in which obtaining and deploying information is meaningful (so multiple paths, engaging puzzles/tricks, etc); and (ii) make transparent decisions in the freeform elements of play, so that players can grasp what is going on, and on that basis make informed moves.
Even if I accepted your definition of railroading, your account of classic D&D here shows that it is not a railroad!
The players
choose to play the game. They know what it entails. Playing the dungeon-crawl game is no more railroading than sitting down to play bridge or chess or
Mastermind (another asymmetric puzzle-solving game).
At this level of description, all that is established is that the players can make some choices about colour. It doesn't show that they exercise significant control over the consequences of the actions that they declare, or the content of the shared fiction more generally.