A runaway train takes you speeding into the distance, but unfortunately does not stop. This means that when you reach your required destination you cannot leave it, but are compelled to be taken further than you wished. The runaway-train fallacy is committed when an argument used to support a course of action would also support more of it. If you wish to stop at a particular point, you need an argument to do so.
It might well be true that lowering a highway speed-limit from 70 mph to 60 mph would save lives. That is not a sufficient argument for choosing 60 mph, however, because lowering the speed limit to 50 mph would save even more lives. And more still would be saved at 40 mph. The obvious conclusion of this run-away train is that if saving lives is the sole aim, the speed limit should be set at the level which saves the most, and this is 0 mph.
In practice the lives at risk for each proposed speed-limit have to be measured against what is achieved by the ability to travel and to transport goods rapidly. Most of our daily activities involve a degree of risk which could be reduced if we limited our actions. In practice we trade off risks against convenience and comfort. If the case for making the speed limit 60 mph is based solely on the lives which could be saved, the arguer will need additional reasons to stop at 60 mph before the runaway train of his own argument takes him to 50 mph, then 40 mph, and finally crashes into the buffers when it reaches 0 mph.
People argue that since, in the UK, everyone has to pay for the country’s National Health Service, this gives the state a sufficient justification to ban smoking, because smokers suffer more illnesses. There may be good reasons to ban smoking, but the argument that the costs of the smoker’s behaviour should be imposed on others is a runaway train. Why stop there? The same argument applies to all behaviour which affects health adversely. It could be applied to the eating of saturated fats such as butter, or refined white sugar. The state could require people to exercise in order to prevent the health costs of their laziness from falling on others. If this argument is to apply only to smoking, there have to be reasons why the train stops there.
Someone boards a runaway train when they are so concerned about direction that they forget to attend to distance. They can continue happily on their journey until their reverie is broken by someone calling ‘Why stop there?’
The state should subsidize opera because it would be too expensive to mount productions without the extra support from public funds.
(And as the train heads off into the distance, wait for the stations marked son et lumière concerts, civil war re-enactments, and gladiatorial displays. If opera is different, we need to know why.)
The fallacy is often committed when someone advances a general argument for something he regards as a special case. If the argument has any merit, the listener immediately wonders why it should be limited to that case. To combat a runaway train, it is usually sufficient to point to some of the absurd stations further down the same line. If good schools are to be banned because they give children an ‘unfair’ advantage, why not prevent rich parents from doing the same by buying their children books, or taking them on foreign holidays?
To lure people on board the runaway train, simply appeal to things which most people favour, like saving lives, aiding widows and orphans and having better-behaved children. Use the general support which such things enjoy to urge support for the one proposal you favour which might help to achieve them, even as you carefully ignore the others.
In a very specialized use of the fallacy, you should gain acceptance of the principles to support a reasonable objective, and only when that point has been reached, reveal the unreasonable objective also supported by the same principles.
You agreed to allow a bingo hall in the town because people should have the choice to gamble if they want to. I’m now proposing to have gaming machines on every street corner for precisely the same reasons.