Hear is what MTH said in a "FYI" letter a few years ago. This was a 2 page letter, highlights below
"How to Detect a Low Battery:"
"When track power is interupted, the sound stops in less than 10 to 15 seconds."
"While running at less than 10 volts on the track, the whistle or horn sounds distorted when activated."
"When attempting to shift from ether forward or reverse into neutral, the sound sputters ---- "
"When attempting to program in Reset, the engine will not lock the changes."
"When the engine locks itself into forward, neutral, or reverse, it will not unlock following the unlocking procedures --- "
"If you are experiencing any or all of the above problems, test the battery as follows:"
1. "Try a quick charge. Place the engine in neutral with the throttle set to 15 volts and the smoke turned off for one hour ---- if it preforms better, continue to charge the battery for a total of 14-16 hours to ensure a full charge."
2. "Install a regular 9 volt battery and test the locomotive ---- If the engine runs properly with the 9 volt battery installed, the NiCad battery has lost its charge. You can continue to run your engine with the 9 volt battery in the short-term while you await delivery of a new battery. "
3. " You can buy a new Nicad battery from MTH or buy a 8.4 volt, 150mAh, Nickel Metal Hydride battery which is suitable for use in MTH engines from Radio Shack, stock number 23-529 "