Merstham Valley Rly added, and also updated the Middleton Rly and added the East & West Yorkshire Union Rly.
Sources of information vary. I started by using old maps and reference books I collected over the years or borrowed from libraries, but more recently almost everything is available online. There are lots of articles, not to mention forum posts about various lines, and many of the published pictures give vital clues to locations. Both Wikipedia and RailBrit contain a vast amount of information, and I've tried to link relevant articles to the tracks. There are also some excellent map sites now available which I'm sure most of you are already aware of, such as
www.nls.uk,
www.sabre-roads.org.uk,
http://www.ponies.me.uk/maps/osmap.html,
www.old-maps.co.uk,
www.npemap.org.uk, and
www.osi.ie
Ultimately it all has to be overlaid on Google Maps, and for this I use Google Earth. The historical imagery in Google Earth is really useful (for instance with the Gorse Blossom Rly), and the ability to overlay maps and images makes accurate positioning possible.
The US maps are harder to find online information for, so I've relied more on reference books and the Google satellite imagery, but luckily disused US railroads tend to still be easy to trace as the landscape changes so little.
Colour coding of lines was always going to be a problem. Unfortunately I'd drawn much of the maps by the time someone pointed out I should have matched it to the Ian Allen sectional maps - D'oh! So apologies if the colours are unfamiliar. Hopefully the colours do at least illustrate how the different companies carved up their territories and why some towns have so many competing railways.
The most requested feature has been for a time-slider or ability to pick a specific year. Unfortunately the maps haven't been designed with that in mind, and it would take a lot of work to redraw the maps and do the research into each line. Maybe I'll get round to it one day, once the UK, Ireland and US maps are completed!
Cheers