Going back to your original post, there is at least one class 55 ("Deltic") registered for main line operation at the moment, that being 55 022 "Royal Scots Grey" in
BR▸ blue. So there's always the chance that one might go whizzing by one day when you're waiting on a platform... Similar in shape to a class 37 but if you compare one next to the other they actually look quite different.
55 022 picIn fact you can often tell a Deltic a mile off (quite literally) because they're fitted with high-revving Napier engines based on a marine design that make a very, very distinctive noise. Not to mention that they make the ground shake even when they're idling...
55 022 videoI'll put my nerd hat away for the moment...