Back at the C15 locomotive, I've been fitting the pipework that runs along the side of the boiler. There's quite a lot of it and working out where it should go has been a problem. Put simply, the pipes emerge from the cab front and end up in the smokebox. The route they take is the issue.
Looking at the prototype photos it appears that no two locos are the same. It's possible that they even changed during the live of the engine. The basic route is the same but fitting seems to have been a pretty ad-hoc thing with many variations on the bends in the pipe. Some look suspiciously like the pipe hasn't seen a proper pipe bender. Most don't do anything for the clean lines of the engine, especially the larger pipe which runs at all sorts of funny angles when it leaves the cab.
On the model, all this is produced using various thicknesses of wire. I always use stuff supplied straight rather than rolled - this job is hard enough as it is without acting like a brass snake charmer !
Another issue is fixing the pipes/wire to the boiler. If there are clips, and I assume there are, then these are tiny and not visible in pictures. I just soldered the wire wherever I could secure it and hope no one notices.