Funnily enough I just spent a couple of days doing the York/Shildon double.
York, so much stuff to see, Big sheds, full of trains, lockers so I didn't have to haul my bag around, the odd comfy seat to collapse in (yay), model railway not running, some of the locos were lacking info panels, FS parked outside out of view, no clock on the viewing gallery, not much to see in the south yard (boo)
Shildon, big shed full of trains with trains outside, well stocked model shop, exhibits with simple laminated A4 card attached with information (yay), some exhibits lacking the laminated card, kid's play area next to the aforementioned snowplough related video meant I couldn't hear it (boo).
All things being equal, a good two days!