At risk of being accused of slipping into a total meltdown over this Nuclear Flask layout concept. I've been looking at what would be needed to operate and build this. Nothing special is required on the loco front, though I doubt the 08 that I have would be seen on such a working. I have a 31 on my wish list and that would be a very excellent loco to work such a train. But most any class of loco has taken a turn on such workings. The nuclear flask and wagon is available from Genesis kits in several variants. Barrier wagons can be anything you have to hand depending on the period of your layout. I have a Railfreight OBA and some 16 Ton Minerals that could pass muster. A Brake van would be needed depending on the time period, again anything goes. If you want an ex-SR "Queen Mary" go for it. They were seen regularly on the Trawsfynydd trip.
The heavy crane is available from Ratio No. RA546, though I first saw the RA545 and fancied adapting that to suit. Take your pick on the buildings. I like Scalescenes and they do produce a superb looking goods shed that is almost Great Northern looking. This layout is really beginning to look a quite easy proposition to build.
Operationally, I guess it can be said to be somewhat limited in scope. But that I feel would be glossed over by the operating loading/unloading of the nuclear flasks. Plus you could also run engineers trains and why not a railfan DMU special? They ran to Trawsfynydd with great regularity in the later years of the lines life.
(I'd also like to point out I have no connection to "the signal box" other than being a satisfied customer in the past)