M.T.H. HO 4-8-8-4 Big Boy Engineer's Manual page 15

Lighting
Your model is equipped with a headlight, backup light, cab interior light, number
board lights and classification lights.
The first thing to know about the lighting is there aren't any bulbs. It's all LEDs so,
if you look for a section on how to replace them, you won't find it. You should
never have to.
As with sounds and smoke, in analog DC mode, lighting operation is fully
automatic. They're always on and behave appropriately. The headlight functions
under rule 17 and dims when stopped for a time or while backing up. Of course,
just as you begin to move forward, it goes bright to show the way.
The cab light is always on so the engineer and fireman can always see clearly!
The tender back-up light only illuminates when in reverse, even if you're not
moving. It goes off when in forward. This is really handy since you'll always know
which direction the model will move next.
M.T.H. HO 4-8-8-4 Big Boy Steam Engine
15