I hate to dive back into this, especially with someone who appears to be trolling, but I think the problem is in definition. A linear adventure is not the same as a railroad. Most published adventures, for instance, are *not railroads*. Here are a couple of definitions of what a railroad is:

From rpgtheoryreview, emphasis mine:
Railroading is a term used to describe the imposition of a predefined set of resolutions onto the choices and conflicts that occur in play by a storyteller or game master. Essentially, it's what happens when a person tries to make themselves the sole author of the story. . . .

Most games have some predefined narrative structure, and there's often an agreement among players that games will contain particular plot types or elements. Railroading only takes place when player actions are prevented from having any effect on the flow of events.
From TVtropes

The answer is called Railroading: In short, the GM takes any measure necessary to ensure that there is only one direction the campaign may proceed — his planned direction. . .
In practice, the use of Railroading is generally regarded as one sign of a poor GM, as forcing the players down a single predetermined path (like cars on a railroad track, hence the name) runs against to the collaborative nature of a tabletop RPG in the first place, where every player is allowed an equal voice in dictating what happens next.
From RPG Geek:

Referring to a game's story being forced in a particular direction most often by the GM
An adventure with a "plot" is not by definition a railroad, nor is a linear adventure (such as the many Pathfinder APs); the problem comes into being when the DM has already determined how the players must proceed *and* forces that outcome to occur, regardless of what they actually do.