(20 Jan 2022, 5:38 pm)BusLoverMum wrote As I suspected, as it looked like an industrial line, just had a look at some old OS maps and the 1947 one has a brick and tile works there.

https://www.francisfrith.com/uk/finchale-priory/maps

Didn't know about the brick works, but makes sense with some of the ponds there.

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(21 Jan 2022, 4:11 pm)GuyParkRoyal wrote The concrete buildings was the Ministry of Supply Brasside Munitions Depot set up to supply the Royal Navy during world war two. The site had 24 concrete storage bunkers and ancillary buildings with a spur from the main railway network. Apparently the link to the main railway network was removed during the building of the prison.
The site covers a huge area and can be viewed on youtube from a drone survey uploaded by John Twist.

Munitions dump! That's similar to the story I heard. 
Mind, the prison was only finished late 70s/early 80s, so that's a long time to leave the spur in place.