Clive Feather:
When you've finished, the grid holds the distance to the nearest
station. Convert it to a GIF and fiddle with the colour map and, for
example, you can have a map where places within 1km of a station are
green, within 2km are yellow, and more than 2km are red.

With suitable travel-time assumptions, you can make an "isochronic map"
like this one, which I occasionally use as a screen background:

http://www.mysociety.org/2006/travel...big-1177px.png

Delete the last component of the URL to see other examples by the same
people, and discussion. Google on "isochronic map" to see examples by
other people.
--
Mark Brader, Toronto | I am a mathematician, sir. I never permit myself
| to think. --Stuart Mills (Carr: The Three Coffins)