This is a more complicated engine than normal to paint - you have to do the inside as well as the outside!
Anyway, here's what's under the body - the boiler unit is painted black, the same Halfords spray I used for the sideframes in fact. I just masked off the motor and gears. A bit of subsequent masking (read: holding a bit of card in front of the spray can) saw it matted down with Dullcote. Next, a dry-brush with rust and Matt 69. Then some weathering powders.
The wooden floor is plasticard painted with Precision natural wood, streaked with Humbrol 64 and metalcote gunmetal. Finally a wash of track colour.
I'm not going to go mad in here - you can hardly see it with the body on top but this is a loco that encouraged peering in to see through the windows and doors.