Author here (Nathan). The goal for this site was to map the entire UK rail network: not just the parts a journey planner surfaces, but heritage lines, freight-only curves, named tunnels and viaducts, and the thousands of stations that have closed and were never mapped.

The project is built entirely on open data. Lines and current stations come from OpenStreetMap via Overpass, closed stations from Wikidata (approximately 6,100 that fall outside a 250 m radius of a live OSM station), and postcode lookups from postcodes.io. The main challenge is that the sources rarely agree with one another, or even with themselves, so much of the work involved small reconciliation rules. For example, the heritage flag is propagated across every segment sharing a line name, so the Swanage Railway is coloured consistently.

I shared an early version with a railway enthusiast community, and a large share of the fixes came from people who know the network considerably better than I do. A full write-up of the data challenges is available here: https://trainmap.co.uk/story.html

No account or app is required. You can search by postcode, station name, three-letter code, or line name. Corrections are very welcome, as there is always a station someone knows I have got wrong.