Diesels at Reading Station UK 1988, Before Redevelopment

In 2014 Reading station was re-opened by the Queen after extensive redevelopment.
From the archives, this film shows the station how it was in 1988, with diesel locomotive traction galore!
It was a noisy place, and if you're into your diesel sounds viewing with a pair of headphones with deep bass is recommended!
At 25:13 there's a lot of acrid smoke coming from a Class 33 on a heavy freight train. We've never been able to work out the cause, but it could be a stuck brake shoe on the engine or some have suggested a smoke bomb prank!
After the train passes the station, in the distance it seems to have stopped. Does anybody know what else this could have been?

Видео Diesels at Reading Station UK 1988, Before Redevelopment канала Rail Flicks