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