Surely its weight would not be allowed over the underline bridges? I have never heard of a loco of that weight and power classification on that branch.
It is/was however recent engineering work done to the track and rail carrying infrastructure meant that the kesr can welcome larger visitors to the line. Hence why 257 Squadron visited last year and the U boat from Swanage for this years gala. Also, Britannia is only running the length of track between Robertsbridge Junction station and Northbridge street which definitely supports the weight
