Except that linear and railroad aren't synonyms. A linear adventure is not necessarily a railroad. A railroad is where the referee negates player agency to force the referee's preferred outcome, whatever the scale (from quantum ogre on up to completion of the campaign). A linear adventure simply has a predefined beginning, middle, and end. If the players willingly follow that line, without the referee negating the players' agency to choose otherwise, it's not a railroad. If the players try to deviate from that line, and the referee negates player agency to force the players to stick within the referee's preferred outcome, i.e. strictly following the linear adventure, then it becomes a railroad.