All the different trackmobiles use the same chassis and mechanical parts.
https://www.lionelsupport.com/...RIZED-48-TRACKMOBILE
You access the gears by removing the cover plates holding the axles in from the bottom. I seriously doubt it is the gears.
What is a far more likely answer and related to this area is there are 4 small brass strips also held with the cover screws that pickup power from the axles and wheels. Highly likely you have intermittent power due to one of those not picking up.
Part number 1 and 2 in the diagram and parts list
Covers 3 and 4 are what you remove to expose the axles and gears, as well as the brass wiper contacts.
Last- one more note. If it's not track power and wheels pickup problems- another problem could be radio signal reception. These smaller TMCC controlled motorized items have limited TMCC/Legacy antenna and so test by placing your hand just above but not touching when trying to run. If you hand improves reception and it runs smoother- there is your answer, it's signal not power and not gears.