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The first thing I thought of was the Beamish Museum with its multiple types of heritage lines, from steam to electric.
Also, in the National Railway Museum branch in Shildon in that same area, it runs a short line running along the original right-of-way of the Stockton and Darlington, the first locomotive-powered railway to carry passengers. They are currently using the Furness Railway #20 locomotive for that service; it's the oldest working standard gauge locomotive in the country.