IN the 1st class cabins in the Thai trains there are power plugs, though not sure about how reliable they are. Not sure about second class.
In addition to the 1st class cabins on Thai sleeper trains I seem to recall seeing powerpoints/sockets on the wall in the Sprinter trains (2nd class aircon seat cars). I'd be a bit hesitant to wack my laptop/iPod plugs into these (ages old) but you can have a go. No trains to speak of in Laos and Cambodia but maybe someone else can reply re. Vietnam etc.