M.T.H. HO 4-8-8-4 Big Boy Engineer's Manual page 10

Next, remove brake shoe detail by gently prying and pulling up the part and set
aside. Remove the 5 screws holding down the rectangular chassis bottom cover.
Carefully remove the cover... then carefully remove the 2 screws holding down the
gearbox cover, set them aside and then stop. Don't do anything further until reading
the next step completely.
Gearbox Screws
Before proceeding, there are a couple of important things to know. First, under
the square bushings on each end of the axle are tiny springs. They are critical to
the proper operation of your engine and you don't want to drop them. Next, look
closely at the wheel set and bushings and take note of their orientation.
Brake Shoe Lever & Bellcrank
Brake Shoe Detail/bottom plate
Gearbox Cover
Bushings
Gearbox cover
removed
Traction tire
Gearbox Cover
M.T.H. HO 4-8-8-4 Big Boy Steam Engine
Screws
Gearbox Screws
Nontraction tire
10
Gearbox Cover
Gearbox cover
Driver springs
Brake Shoe Lever & Bellcrank