This only seems to be standard overground trains. If you add in metro networks like the London tube, or light rail / trams like in Manchester, then you’d get at least hundreds more.
The map includes metros across the Tyne and Wear Metro in NE England, and while its not perfect, it's by far the most useful train live tracking I've ever seen. There's quite a few places in the UK with different rail systems that don't fit together (and have apps of varying quality/usefulness)