It’s generally invisible except through the windows. So I don’t want to spend too much time on the cab interior and specifically the backhead. A modeller’s sketch is sufficient, I think.
If I recall, the backhead for #10 took an enjoyable day of modelling. It was styrene plus a few pieces of lost wax jewelry representing what I surmise would have confronted the fireman and engineer when they stepped onto the footplate.
On 622, I plan to 3D print the backhead so that once she’s built, I can quickly put together her sisters as well. Because it is printed, I am limited to about 0.5 mm for un-supported cross-sections. So, everything is a little chunkier than I might like. However, it should be fine through the windows.
It hasn’t taken me just a day to put together though! I think I’ve been drawing valves and pipes all week, and there could still be more. Thank goodness I’m modelling a little tea kettle, and not one of the big super-power monsters of later years. Either way, I think if I were doing it again, I might just make the valves, and run the pipes with soft wire.