You can't railroad a willing person in my opinion.
Railroading is the deliberate act of limiting or eliminating choices and the players knowing that their choices are being curtailed or that their choices, whatever they are, lead to the same point.
The willing person who is following the plotline or the bread crumbs to whatever lies ahead is not being railroaded, whether or not he had options or not because he is choosing to follow the plotline and not exercising a possibility of doing something else.
Railroading is when the players choose to depart from the established plotline and the DM takes actions for 'force' them back on.
Case in point.
The DM has a dragon encounter he wants to run. The characters can get forewarned of the encounter by asking the right NPCs they encounter.
Outcome 1: The characters find out about the dragon but willing decide to proceed towards the encounter anyhow. The characters are not being railroad as they still have free will and have freely chose to face the dragon.
Outcome 2: The characters don't ask questions and aren't aware of the dragon and proceed unawares to the encounter. The party is not being railroaded. The party not exercising a choice because they are not aware they have one is not railroading. Their choice is to continue, not aware that a dragon awaits, therefore, choices have not been taken away.
Outcome 3: The characters find out about the dragon in the hills ahead and instead, choose to head to the forest in another direction, only to be attacked by the dragon overflying the forest. The party is being railroaded now since their choice was to avoid the dragon encounter and the DM hit them with the encounter anyway. The party exercised a choice but the DM did the dragon encounter anyhow.
Outcome 4: The character find out about the dragon in the hills and head for the forest, only to run into a dragon living the the forest. The DM says it is a different dragon but the party didn't want to mess with any dragons, so it's a railroad. Again, the party exercised a choice but still got the DM's dragon encounter anyhow.
Outcome 5: The party finds out about the dragon in the hills and instead, head for the forest, only to find that the forest is on fire. They decide to back track, only to find that a entire horde of orcs is rampaging through the areas. They make for another location, to find that the weather or other conditions preclude them from going that way. One by one, all options are closed down do that the only option remaining is heading for the hills and the dragon. This is the classical railway. Party cannot exercise any choices because all options are shut down and the only choice is the what the DM wants.
In short, a railway is when the party is aware of something they want to avoid or not deal with at the moment and through lack of choices or manipulation of choice outcomes, all paths lead back to the very thing the party has made a choice to avoid.