Batteries and a computer are mounted in every train. They are recharged every cycle using bus bars in the station. The new sleds for Matterhorn Bobsleds were a HUGE design dilemna: how do you fit bench seats, automatic locking system for seatbelts while keeping the weight the same or close to the original? That's where things like padding get cut sadly and why the current Matterhorn sleds are so uncomfortable.
On the aspect of weight, take a look at this:
Those were the original Indiana Jones et le Temple du Peril trains at Disneyland Paris. They had 4 seat per cars and 8 per train total.
Those are the current trains that were added in 2000. Notice something is missing? Padding. You went from sitting on comfortable foam padding in the original to having a hard headrest and two thin strips of foam for "comfort" as you sit on the fiberglass directly. They had to do this as they increased the capacity by having 6 person per car (12 per train) and having to keep the weight as close as possible to the original.
Going back to Space Mountain, there is no way to add the heavy on-board audio system to the existing cars or designing new trains with it that would not greatly increase weight. The current track is in so bad shape they had to install magnetic brakes a few years ago before the big drops to protect the curve and helix at the bottom. Heavier cars would certainly take us to a Disneyland circa 2003 situation.