An adventure can't really be a railroad in and of itself anyway, as "railroading" is something that someone has to do to someone else. A DM can railroad the characters even in a sandbox style of adventure ... look at how many DMs come on the internet to ask how to restrict their players' access to the temples in PotA out of a fear that the PCs will get TPKed.
As long as everyone knows that they're playing a linear, plot-based adventure like Tyranny of Dragons and agree to go along for the ride, then there is no railroading. It's only if the DM insisted on following the plot, regardless of the choices that the players make during play, that any railroading would occur.
I'm sure some players would cry foul if the DM sprang a "you're in prison" start on them, but as I said above, if the DM gets the players' buy-in ahead of time, there shouldn't be any problems.
I think the issue of railroading generally rears its ugly head when the DM turns the campaign's plot into a guessing game or makes it a "gotcha" surprise by not telling the players anything about what they're in for. I personally have veered away from that approach and am more open about what kind of adventure I'm running with my players, so they can make informed decisions about their characters and the like. If I'm going to run a campaign in Ravenloft, for instance, I'm going to tell my players that, so they go in knowing that there will be elements of horror and the like in the game. If I'm going to run a campaign-length adventure like OotA, I'm going to tell them that - and I'm going to get them to choose some of the campaign-related bonds - so that they know to make characters that will be invested in seeing the adventure all the way through.
Openly communicating with your players can significantly minimize the risk of having accusations of railroading thrown about. It can also minimize the chances of hearing the players ask, "Why are we doing this?"