Steve,
The HSE did publish a guide a few years ago and the recommended minimum distance between tracks used for public passenger carrying is stated as 2m, although from memory it's not clear whether that's between running edges or centres.
I know of one railway that has it's tracks closer than 2m and they close one line if they are passenger hauling.
Eddie
Edited By 61962 on 03/07/2013 22:43:04