Before I start, I think there's bad railroading and good railroading.
To me, before it's railroading at all, there has to be:
- Only one choice; and
- An artificial constraint; and
- Not the consequence of the player characters' own actions.
For example, a dungeon might contain a choke point. A number of dungeons only have one entrance, for example, or only one staircase between levels. To me, that's a geographical choke point, not a railroad.
It becomes a railroad if the player characters have no other choice but to pass through it. (So for example, the player characters are forced into the dungeon because the King's ordered them in, and there's a bunch of armed guards directly behind them to force them into the dungeon, then a dungeon with only one entrance becomes a railroad.)
But, that only applies if it isn't the player characters' fault. If, during the previous session, a PC stood up and, in front of the Queen and the whole Court, asked the volatile and bad-tempered King to explain why he has three eighteen-year-old mistresses in town, for example... then the King orders his powerful guards to take the player characters away and throw them in jail... then that doesn't count as a railroad to me because it's a natural consequence of the players' actions.
"Bad" railroading is when this happens during play. So to me, the bit in A3 where the players are captured because the plot requires it is "bad railroading" -- particularly since it deprives them of the climactic battle with the evil Slave Lords themselves, when the Slave Lords are directly in sight!
The bit at the beginning of G1 where the player characters are given a reason to go kill the giants, told to go kill them, then suddenly appear near the Steading of the Hill Giant Chief,
is a railroad but it's used to shift the players quickly to the beginning of the adventure where they can start making interesting and tactically valuable choices.
Thus it strikes me as a railroad that's relatively less unpleasant for the players than one that happens during play. Hence, "good" railroad -- effectively a taxi-ride to the start of the adventure.