What folks are saying is MTH Proto Sound (very earliest versions) and MTH PS-2 5V and 3V types all need a fully charged good battery to run in conventional.
Product number can tell us which model you have. If you have the directions, you can change the battery to a 9V type if it is PS-1 or 5V type engine. This can give you a quick test if it is the battery. Once you know it works, replace the 9V with the 8.4V Rechargeable type. If it is a PS-2 3V it will have a 2.4V AA type composite battery. That you can't test without a new 2.4V Battery. G