4-4-0 American

Overview:

Apparently 2 revisions to the drivetrain.

Original has round driving axle, and the gear has a round hole

Second version has a "D" shaped axle where the gear goes. (Picture by PeteThornton)

Model: 813XX-13

Shown below, the version 2 gearbox

https://www.mylargescale.com/threads/bachmann-spectrum-4-4-0-and-2-6-0-gears.89474/#post-1147288

https://www.bachmanntrains.com/home-usa/board/index.php?topic=19904.0

https://www.bachmanntrains.com/home-usa/board/index.php?topic=19904.0

https://www.gscalecentral.net/threads/bachmann-spectrum-gear-replacement.313971/ 

https://www.bachmanntrains.com/home-usa/dwg/dwgs/LS4-4-0DCWIRING.pdf

Removing the bottom cover:

Be aware that the chuff contacts/wires are attached to the bottom cover. The wires are normally pinched/hung up "above" the motor block, so you normally have to remove the motor block from the boiler, then you can un-pinch/hang the chuff wires, and now pull the bottom cover away.

Drive gear replacement:

As with many (actually most) Bachmann steamers, the drive gear on the axle splits and spins on the axle.

Here's a video from Steve Sidensticker (a SDGRS club member, has an operating layout "group", and does battery power installs: