mjh

Just to be clear, that isn't a flask, or a container. This is the hood mechanism that goes over the flask. The triangular piece at one end is the fixed / latch end, the rest of the hood slides away so that the actual flask (that sits within - and below!) can be removed. There is a good picture here https://www.railwaygazette.com/freight/nuclear-decommissioning-authority-orders-flask-wagons/41734.article showing the hood slid right back and the flask being craned out from within. Your model railway your rules of course, but it's not an equivalent representation of how the things work.

Australia has gone the same way actually

Allocations at 1994 per On-track Plant on British Railways, SCT publishing: 81503Rugby 81504Rugby 81505Temple Mills 81506Peterborough 81507Hither Green 81508Three Bridges 81509Reading 81510Reading 81511Woking 81512Healy Mills 81513Eastleigh 81514Cardiff 81515Low Fell 81516Reading 81517Three Bridges 81518Crewe 81519Hither Green 81520Exmouth Jn 81521Doncaster 81522Temple Mills 81523Bristol Marsh Jn 81524Rugby 81525Temple Mills 81526Kings Norton 81527Healy Mills 81528Peterborough 81529Nottingham 81530Crewe 81531Kings Norton 81532Ashford