Before I had all the problems painting the outside of the locomotive I'd painted the inside of the cab while I could still easily get a paintbrush in there. There are two parts to this, painting the backhead controls and painting the inside of the cab walls.
The first job was to touch in the details on the backhead and for this I used three paints. I gave the main backhead surface a light dry brushing with RailMatch oily steel to help bring out some of the details and I used Humbrol acrylic buffer beam red (number 406 from their Rail Colour range) for the regulator handle. For the brass I used Alclad 2 polished brass lacquer. This isn't something I'd used before but it gave just the effect I was looking for. You are supposed to airbrush the lacquer over a gloss surface to give a high shine finish, but I brush painted it over a satin surface which gives a more aged look. I used this lacquer rather than a paint for two reasons. Firstly I couldn't seem to find a brass paint in any of the ranges my local model shop stocks, but more importantly I'm not really a fan of using metallic paints at this scale, unless they are being sparingly dry brushed. I find that the shiny flecks in the metallic paints (that give them the metallic look) are always fairly large and just look really wrong at the small scale we are working with. This lacquer doesn't suffer from this problem at all and I'm betting it would look fantastic airbrushed onto a larger area.
The second job was to paint the inside of the cab walls. From photos, and from having visited the odd cab over the years, I've noticed that generally the bottom half of the walls are painted black, while the top half is usually a buff or cream colour. I'm guessing the use of the lighter colour is to make the cab feel bigger and less claustrophobic. Given the paints I had to hand I used ivory and black from the Model Color range; painting the ivory on first and then masking to get a straight edge to paint the black against.
Personally I think both parts have worked really well, and now that they are assembled together it really does look good.