Useful links and sites
Maps
OpenRailwayMap
What it says...! A full zoomable/scrollable map of the UK railways.
https://www.openrailwaymap.org/
New Adlestrop Atlas
A PDF file atlas of current and historic railway lines
https://www.systemed.net/atlas/
Library of Scotland Maps
Side-by-side and geo-located (overlaid) maps, with many selectable layers including 1st edition 25inch OS maps - covers all the UK.
Railmap Online
Modern map overlaid with historical railways (including mainline, industrial, mineral lines plus street tramways and more).
https://www.railmaponline.com/UKIEMap.php
Other Railway Information
Railway Codes
All sorts of stuff about codes used in railways and railway signalling systems.
http://www.railwaycodes.org.uk/index.shtml
Railway Signs and Signals
Details about current and historical signs and signals on the railway systems.
Tampere Railcam train information
Site showing upcoming trains at Tampere railcam.
https://raiteilla.fi/railcam/services.html
Railway News
Railway Herald
Rail news for the enthusiast sector
http://www.railwayherald.co.uk/
Railway Ticketing
GB Rail Rover Guide
Independent guide listing Rover Tickets and Railcards available
http://www.railrover.org/index.html
Oyster Fares
Independent guide about usage and quirks of Oyster Cards in London area The Information pages are very useful to not getting overcharged for some journeys you may make.