Protect disused railways by stopping residential and commercial development on them. Leave them be as a future-asset. Britain's transport needs could require cycleways to be built and train lines to be reopened, so these assets should be preserved.
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The United Kingdom is criss-crossed with disused railways, that provide arteries through our cities. Some have been extensively built over; others, not yet. By leaving them unmolested, there is still the option of restoring them as railways, or turning them into cycleways, in the future. Let us keep our options open, and not build over a future asset.