Jon, the Getty image is of a Stockton and Darlington composite carriage, No.31.
I have tried to discover a bit more about the why and wherefore of the photograph, but have had only limited success. It seems likely that it was taken following it's restoration in 1875 for the S&DR 50th anniversary exhibition. There are three splendid gentlemen in the picture, and one of them could possibly be Mr. J.B. Harper, Assistant General Superintendent of the North Eastern Railway. He was passionately interested in the history of the S&D, personally collecting many related artefacts and relics, and was ultimately responsible for establishing the very first railway museum.
I understand that carriage 31 and also a similar no. 59 both survive intact, and are therefore potentially available for measuring up!
Pete.