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Can someone point me in the direction as to where I can obtain vector layers for the following UK railway lines:
I'm not sure these sources have exactly what you are looking for, but they may be good sources:
They might have what you're looking for and you can use Overpass Turbo, Planet OSM, Geofabrik to download shapefiles or whatever format you need.
A search on data.gov.uk for rail returns several datasets that you might find useful.
For example Network Rail - railway network Inspire data
There's a WFS and a WMS for the rail network and for stations
Here's a preview
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