I haVe found that the best games I run begin with me begining an area with a lot of possible adventures, a strong adventure hook for a "big" dungeon that the PCs know they cannot tackle starting out, and a strong adventure hook for a "maiden voyage" adventure. But the area is literally seeded so that the PCs can go in any direction and seek out something. This is, IMHO, one of the real draws of the Wilderlands setting -- PCs can wander about, involving themselves in all sorts of things.
That said, I do not believe that anything that takes control away from the PCs is railroading. That is simply too broad a definition to be of any value. I do not believe that charm spells constitute railroading (by forcing PC action) any more than I believe paralysis is railroading (by forcing PC inaction). That sort of reasoning results in PC death becoming lumped into the inflated term "railroading". Obviously, the DM is forcing me to make a new character.
If you decide to try to steal the cash box in a crowded bar room and get caught, and then face the consequences, it is not "railroading" in the sense that term is typically used in. It's a bad situation, sure, and if they cut off your hand or leave you to languish in prison it's a worse situation. Freedom of action includes, perforce, the ability to take actions whose consequences curtail your freedom.
The DM saying "I am going to run Age of Worms for those interested in playing it" is not railroading. The players have a choice going into it; they know what they're getting, and they have tacitly agreed to it by playing.
The classic dungeon traps include sliding walls that force a party to discover a new route, and pit traps that send parties to a lower level of the dungeon than expected. To my mind, these are not railroading. One can easily imagine a whole campaign revolving around finding a way out of a massive dungeon. The WLD is centered on this theme, and back in my 2e days I considered running a game where PCs started at the bottom of my mega dungeon and worked their way up (I had whole cities in there). These are perfectly valid campaign possibilities, and the fact that they limit PC choices to some extent does not mean that the game itself becomes a linear progression predetermined by the DM.
Being sent on a mission under some threat isn't railroading, unless the threat is managed in metagaming (i.e., there is no way for clever players to eliminate the threat, there is no way to even try to avoid the mission and run for the hills, and there is no way to simply accept that whatever is threatened may befall the PCs and stoically refuse).
While the plots in Edgar Rice Burroughs' novels are often transparent, they are worth examination in terms of this discussion because his characters frequently face situations that would be considered "railroading" by several posters here if they occurred in adventure design, but which his protagonists deal with in ways that defuse the "railroading" aspects. Tarzan is captured and threatened many, many times. None of these times dismay him, nor do they prevent him from using his considerable skills and wits to prevent others from controlling his actions. If they do have a temporary advantage over him, he refuses to give into their demands whenever possible and bides his time until he can get revenge.
Doctor Who (especially the older series) offers many more examples of situations that, in the typical RPG session, would become unmitigated railroading, but which the Doctor and/or his companions turn instead to their advantage. Conan never sulked in the corner because his avenues of choice seemed limited; like Tarzan, he sought out the best avenue or tried to do the unexpected. Frodo's quest inexorably led him to Mount Doom, but he had the choice to abandon that quest and live (or die) with the consequences.
When the DM thwarts you from trying to seek out the best avenue, or prevents you from doing the unexpected, that's railroading. When the DM metagames in new consequences above and beyond those that would logically arise, not because the world is reacting to the PCs, but because the DM is trying to keep you to the plan, that's railroading.
All games are linear, regardless of who is driving. If the game includes dungeons A. B. and C, and the players choose to go to B first, then A, then C, that B-A-C path is still linear. If the DM sets up A so that it can be survived at low levels, B so that it can be survived at mid levels, and C so that it can be survived at high levels, gives the players the means to learn this, and then lets them decide which order to tackle the dungeons in, that isn't railroading. Not all options have to be equally good. Some players may choose paths that go back and forth between the dungeons as their skills grow. Even low-level divination spells, properly used, can allow PCs to tackle potentially dangerous areas by giving them the means to avoid or specifically tailor the means of defeating an encounter.
In conclusion, if the DM is upfront about running an adventure path, and the players agree, then it is not a railroad. If the DM is upfront about running a series of adventures, with the time between occurirng offstage, and the players agree, then it is not a railroad, even if the DM picks the adventures, and even if he decides you start with no equipment at the start of one of them.
If the DM claims to run a free-form campaign, but insists that adventures be followed in a given order, and refuses to allow the PCs to see what is over the Hollow Hills instead of going to the Tomb of Bad Plumbing, then it is a railroad.
Which is a long way of saying that, in part, I agree with Quasqueton and in part I agree with rounser.
Just my $.02, of course, and IMHO.
RC