Last night my kids & I had been laying Lego Duplo train tracks. We replaced the batteries with a fresh set - ready to go. When we started the train, it moves a few inches and it stopped. Hitting the pedal remotely from the app also faced with the same problem.
It appears that one of the back wheels is loose and getting out of sync. Thus the train became wobbly and lose traction which in turn signal the train sensor to stop.
Man, this is quite a setback for the kids on the first day on lockdown. Checking on BrickLink,= replacement unit will cost USD25 and only available in Estonia.
Legos are built tough to withstand abuse from kids and toddlers so it must be a rather simple problem so I decided to crack it open. Apart from the two Philips screws holding the battery door there's 6 star screws to open.
There's no glue holding it together so you can just set aside the cover revealing almost all the components. The only thing hidden are the speakers underneath the PCB in between the front wheels.
Then carefully take out the back wheel assembly inspecting the gears.
I found that some dust had gunked up the wheels so it's just an easy pick. Grease it with some lubricants and it's good to go.