Last week I talked about the two forms of the 'railroad' and how dangerous one of them can be. Railroads, in general, are looked down upon as an inherently bad thing. However, as with most things in life, there are times when railroads are a good and positive thing for a game. Similar to how a game table has snacks other than Doritos and Mountain Dew, there are circumstances when a railroad is exactly what a gaming group needs.
Here are a couple of times when a railroad is advised...
Poor Group Initiative
Some players simply don't have good self-motivation when it comes to a campaign. They hem and haw and wander aimlessly for far too long. They begin to get upset by the lack of direction. Some groups need a strong lead and the best way to do this is with a railroad. A railroad gives clear goals and an avenue to accomplish those goals. It is better to run a railroad than to let the players flounder.
It's the Journey That is Important
For some groups the ends are not as important as the journey to get there. For those groups the plot is irrelevant and nothing but the "excuse" to go on an adventure. They don't care how they get to an adventure, just that they are in it. Often these groups enjoy the challenges of the encounters, seeking to overcome them as a purely mental contest. Some only care about the vistas of an encounter; how they get there is less important than fighting titans in cloud castles. For these sorts of groups providing a non-linear setting could actually get in the way of their goals. If the choices in a campaign's direction is left up to them they may wander too much, not reaching the exciting encounters they are looking for. For these groups a railroad of encounters provides enough of a framework to provide incentive to the encounters but also keeps the encounters coming for the group in a timely manner.
To sum up, for some tables, it's okay to run a railroad. Sometimes it's the right thing to do.