Steam Along Thomas Coupling broken

Unless I am mistaken the drive gear on the final output axle doesn't appear to be freespinning as is most common...
Take a look right here in the video, looks like it isn't slipping but runs when the wheel does to me.



You have to look close, but you can see two or three of the teeth in the little gap.
It looks to be stationary even when the motor is running unless you move the wheels by hand. I had similar symptoms with a post-'98 Japan Thomas recently, could get it to catch and run for maybe a full wheel rotation and it would usually catch but forcing it past the "clutch" two part gear that drives the axle would sometimes let it freewheel a short distance with gears engaged (switch on) but no battery. My guess is its this gear

Here's what the one out of the problematic gearbox looks like. Note the wider gap between teeth on the left side.

This is the gear you noticed doesn't connect with the previous worm gear - that's true, and lets the train freewheel when turned off, until pushing the on switch also pushes it into the geartrain (and is also how you can get engines to "idle"- pushing the switch just far enough to turn on the motor but not far enough to engage the gears, so the engine hums and sits in place).

Steam Along Thomas uses the standard Thomas/Percy type drive unit, so you could either swap gears from another engine with more issues or if you end up getting a second Steam Along for parts you could swap the whole drive unit.

Regardless, I think we've mostly ruled out electrical problems and its an issue with a cracked gear somewhere in Annie's drive unit.