Welcome to the enthusiast pages. "75F" (Tunbridge Wells West) is a unique survivor, being the only London Brighton & South Coast Railway engine shed still performing its original role.
In the days of British Railways, all engine sheds across the country were given a code by which they could be readily identified in telegraphed and written communications. Principal engineering works and large depots were given a number between 1 and 89, followed by the suffix "A". The smaller sheds in the area were then allocated the same number, followed by a B, C, D and so on. Brighton was nominated 75A, and Tunbridge Wells West, being one of Brighton's sub-sheds, became 75F. Tonbridge, by contrast, was 74D, being a sub-shed of Ashford (74A).
This number was displayed on a cast iron plate on the engine's smokebox, giving railway staff an at-a-glance reference as to where any given engine belonged.
It is a source of great pride to the Spa Valley Railway that many of our engines wear "75F" on their fronts, although you might see 73A (Stewarts Lane) and 73C (Hither Green) if you look closely at some of the diesels...