Just for reference I dug up a schematic and non-schematic map of the UK rail network: https://www.mytrainticket.co.uk/project/uploaded-media/great... / https://www.thetrainline.com/cms/media/2922/tocs-v52tl-decem...

They'd probably need to be combined with a map of population density and nature reserves to produce a "rail planners map". You can already see where the "empty" areas of the UK are already, like the Scottish borders.