. Clear space ahead.
The word headway is used in the rail industry to refer to a safety margin between trains travelling along the same track. Well, it is used here in the United Kingdom's rail network - I can’t speak for the rest of the world. Headway is a measure of time. It is a measure of how long must elapse before the following train may pass the same point as a prior train.
Why use headway?
It is a safety measure, designed to separate trains, and decrease the possibility of two trains occupying the same space at the same time. This is like the three second rule for following distance when driving a car.
How is headway enforced?
As the train passes a signalling point, the signal turns red, and stays that way for the specified time. Following trains are not supposed to pass the signal until it turns green again. Passing when the signal is red is refered to as SPAD or Signal Passed At Danger. Some newer trains may have a ATP (Automatic Train Protection) system that prevents the driver from doing this.
How long are headways?
On UK main line trains, headways are often 1 minute or 2 minutes. But it depends on the location.
On a rush-hour busy station, the 'train right behind this one' will enter the platform a few tens of seconds after the prior train leaves. If you look carefully, you might see the little light turn red and then green again. It seems to me as if the London underground operates on a headway of around 40 seconds. (Ok, I timed it at roughly that. At one station. On one particular day.) But headways at signals between stations may be longer.
What are you, some kind of trainspotter?
No, it is my current job to know this.
So, what's a headway?
About 10 kilograms.
thanks to Sighmoan for this brilliant joke.