Being told "that is not how the game is going to be played" is railroading a player. Others in this thread would undoubtedly agree. You seem keen on ignoring that part of it and want to insist the GM only wants the RR to "familiarize himself" with TWD.
I very strongly disagree with your defining this as railroading. Setting up the game that is going to be played, the genre and setting, the style of game, the characters allowed is all part of what a GM does when pitching a new game. That's not railroading, that's simply the starting point.
If I'm running a fantasy game without gunpowder, it's not railroading to not let someone play a Pathfinder Gunslinger (even if it's in a rulebook). If I'm running a superhero game where everyone is a genetic mutant, it's not railroading to prevent someone from playing an alien or extra-dimensional being (even if they're both common to some types of comics).
Or to put it another way: If I'm running a Vampire: The Masquerade about politics, it's not railroading to tell a player that major intentional violations of the masquerade are not part of the game that's being run.
The problem here is not that the GM is railroading the OP. The problem is that the GM declared what type of game was being run and the OP had no frame of reference for what the game was about. The OP has stated in their posts that they were basically looking to play a typical zombie video game or even some of the hack-n-slash zombie movies, which is really not what The Walking Dead is about at all.
The fact that the other people at the table were "up in arms" over what the OP did in the game says that the rest of the table is not cool with what the OP did either and are probably on board for what the GM is trying to run.
That's not railroading. Everyone else is playing one game, the OP is playing another. They're all creating one story, the OP is trying to play a video game in the middle of it. It doesn't mesh, conflict occurs, and to keep the story running the way the majority want it, the GM gave the OP some material to try and explain what they were looking for.
That leaves the OP with two reasonable choices: Adjust to the game that's being run or sit out until the next one. Just as a player who really wants to play a Jedi has to wait until someone runs Star Wars, someone who really wants to play what truly amounts to an evil character may have to wait for a game where evil is allowed.