I have a feeling there has been some confusion here. Wellington Barracks is a very famous British Army barracks in Bird Cage Walk and has absolutely no connection with the railways. The Royal Scot was a class of steam locomotives built for the London Midland Scottish railway (LMS) in the 1920s/30s. At first many of them were named after British Army Regiments - which is probably where someone has got confused. I would think your best bet would be to write to the National Railway Museum in York, even if they do not have the particular records you are seeking they may know where they are preserved - assuming they have been. Not particularly important but it is obvious you are American - English never referred to an "engineer" on steam trains (or even modern diesels etc.), they would refer to the "driver" and "fireman". Hope this is of use.