#2: Forced Direction. The most common form of railroading lies in creating an environment where the characters really have no choice other than to follow the given path. You come to a beach, there is only one path, the path leads to a waterfall surrounded by cliffs, under the waterfall is a cave, in the cave is a dragon, the encounter ends with a dragon. Though an extreme example, you’ve basically railroaded them into the dragon encounter whether they wanted to do it or not, leaving no options to move to the left or right, sneak up on the cave or find it on their own. Generally, this type of railroading makes the players feel as if they aren't actually playing an RPG, but rather listening to a story that they happen to be intermittently involved with. Try to avoid this type of scenario.
#3: The Railroad Switch. A less efficient, but far more player friendly approach to driving the party to the waterfall where the dragon lies, or the end of any adventure, is to blanket the area with choices, all of which ultimately end at the water. Allow for multiple paths on the beach, these paths lead through different terrain, but meander toward the waterfall, above it, before it, to the left or right. No matter which direction they take they wander into the same scenario. The players have made multiple choices and arrived at the destination you wanted them. They are in charge of their destiny... well, they feel like they are. And you get to keep you game intact.
#4: Conducting. A freer flowing approach, and one I favor as I GM largely outdoor adventures, is make the object mobile. You can fix it in place at some point, but if the players wander far afield just move your encounter and place it on the path they are on. Its usually very simple and costs you only a little in re-imagining the encounter and its place in time. The players are none the wiser unless you tell them, your game stays intact and you really haven't herded them anywhere, you’ve simply dropped danger in front of them.