Dropping off train it has banked through the Severn Tunnel

Peter Kerslake says "6100" class 6158 had been allocated to Old Oak Common within the mid 'fifties, but here she is pictured having banked a freight through the Severn Tunnel from Wales and now dropping back from her train.
A number of these banking locomotives would often then run forward ahead of her freight and act as a pilot engine for the remainder of the climb from Pilning up to Patchway before then uncoupling and returning back to Wales through the tunnel with the second of the two brake vans usually attached to such freights, two specially adapted brake vans being required by the strict safety regulations imposed on such freight workings through the tunnel."