The NRM needs to appeal most of all to the youngsters of today. Those who look at the items or the wonderlab (whatever that will be) and think i really want to get involved with engineering and then have a lifelong enthusiast and interest in the industry.
At the moment the railway museum really just is a room with some trains in and there isnt really much of a story/route through, its just a random assortment of trains. York is a massive tourist destination for families and the museum needs to capitalise on that. It doesnt need to attract the enthusiast, they will most likely come even if they hate it.
It needs to be bright (natural light) and be interesting. This doesnt mean the national collection cant have other large exhibits, but who really cares about Class 40s, 2-BILs, Class 31s. These are not items that leave an impression.
The problem with most diesels/electrics are that they are essentially just boxes on wheels, whereas there is normally a lot more to see with a kettle.