Railroading is theft of player agency.

Everything in D&D is cooperative. Yes, the DM creates the world, and runs the game, but if he's not creating a world the players want to be a part of, or running a game they enjoy, he's not doing very well. It's obvious when getting a group started, picking an edition, a setting, deciding on the tone, who will be the DM, what things are/aren't allowed, etc, but that cooperation persists throughout the entire game.

Yes, the DM can do a lot of planning, and printed D&D modules have always been "planning in a box", but you've still got to have that cooperation with the players. If the players don't want to follow your hooks, you've got to be ready. If you can weave your preparation around their decisions and everybody's happy, then that's just clever DMing. It's when the DM forces events to happen in spite of the players, that's when you start to get into railroad territory.

Yes, the DM is expected to do all of the planning work, and is designated as "in charge", but it's still a cooperative experience with the players, and no amount of planning means the players have to do what the DM is prepared for, you know? I would not put so much emphasis on the planning part of it. You say "good encounters are planned", one of my favorite sessions could have been summed up on a notecard. "Local shady noble is witnessed receiving a package form a shady contact. He's having a party this week, the players have the means to attend. He's going to summon a demon at the party." And I bookmarked the monster manual page for the Barlgura, and that was 100% of the planning I did. Everything else I left up to the players, and it worked out splendidly.