Lively Polly at Wellington Dock on the Overhead Railway. The bridge behind was the L&Y Rly High Level Coaling Railway to Wellington and Bramley Moore Docks.
Amateur film of the LOR from the NW Film Archive: https://player.bfi.org.uk/free/film/watch-liverpool-overhead-railway-1893-1956-1956-online?fbclid=IwAR0oXF3dJZ0duR1nSFFF4hVqVf13NHzEq8eo9t1qSMfGqpkEQa5ypJKGnHI
The LOR employed a Ruston for engineering work trains, replacing Lively Polly. Information on colour/livery seems non-existent? So perhaps something like this image is a reasonable guess? Copyright Idle Drifter, photographer unknown.
I was pleased to see your York photos on social media, including the GOG. This is a very unusual project and I can see why it appeals to the crowds at Shows. It's been interesting to follow progress over the long term and I look forward to catching up with the diorama at a future show.
A Jinty hauls a rail tour towards Riverside Station while a Pug trundles along the MDHB main line with a freight transfer between the BR goods depots on the other side of the dock road.