How do you do that? I'd be very interested in copying.
The 'simple' answer is I downloaded the
Strategi dataset from the Ordinance Survey. This is open source and available for anyone to download and use, all you have to do is indicate the data is crown copyright.
Without going into too much detail, I imported the data into a GIS package (I use QGIS). While this software graphically represents it as a map (you can also download the pre-set OS 'style' which saves a lot of time) there is also a data table underlying this. So for each section of railway (usually between stations, but even this can be split into many sections) there is a dataset which lists a unique id code for each section, its co-ordinates, tells you what sort of railway it is (e.g. heavy rail, light rail, tunnel), etc. Think of it as a spreadsheet which includes co-ordinates to allow GIS software to map the data. I added a column to this table which allows me to mark a section of railway with an "X" once I have covered it. T I then tell the GIS software to colour all railway lines labelled with this "X" to blue, and all those without the "X" to light grey.
To produce the output on my coverage map, I disable all but three layers (coast line, railways (obviously) and urban areas), as everything else incuded in the data such as roads, rivers, town names, etc. are irrelevant and if I included the map would become rather busy!
Sounds simple eh?

The reality is it is much harder than that. I taught myself to use the data and the software, and have quite a bit of experience in using it for a range of different interests. I also had a bit of a head start as I occasionally had to use GIS for a previous job, and was given some basic training to get me started. But don't let that stop you! Once you get the basic principles of how it works it's actually not that difficult to master. The point is all it costs you is time.