I am a bit late to the party, but I did want to comment on this. Generally, the trains wouldn't have batteries, but rather large capacitors that would hold the electricity needed to run the speakers. There really isn't too much to the recharging of the capacitors on the train, they just run a energized rail in the loading station that charges the capacitors aboard the train. The capacitors allow for lots of current when needed and can charge and discharge quickly.
And yea, you can get tiny amplifiers these days that are actually quite powerful (the Harry Potter ride at Universal uses Bose amplifiers onboard that you can hold in your hand). Same with subs. Compared to even 10 years ago, audio technology has really improved.