Although new to you this engine has been around since 2000... The battery will need replacing for sure . It's probably one of the early white ones... .. You could try a 9 volt alkaline for testing but don't leave it in the engine.(it's probably in the tender)
Proto-2 engines require a good battery to leave neutral and have any resets stick. Yep a BCR is good as well.
Nice solid engine made strictly for 3 rail.