Someone has probably suggested it before, but the iconic British Rail Class 158 ‘Sprinter’ would be a great loco to have. They’ve been around for decades and there probably isn’t anywhere they haven’t been, much like the British Rail Classes 140-144. As for routes, there’s a huge list of possibilities. But my northern bias says bring them home! Perhaps something like Wakefield Westgate - Knottingley via Wakefield Kirkgate and having the Leeds line from Knottingley to Castleford, and Sheffield - Leeds through Wakefield Kirkgate. I believe this triangle of stations would have enough complexity for a varied timetable given the relative short distance on the longest route. Or for something longer maybe Hull - Scarborough, or Newcastle - Chathill.
DLC could include any of the following, Class 140, 141, 142, 143, 144, 195, 331, 333 though the latter ones are more modern.
