4-4-0 American

Overview:

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.gscalecentral.net/threads/bachmann-spectrum-gear-replacement.313971/ 

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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: