Russian sleeper trains are fine. Just be sure to have plenty of booze and food to last the journey. I suspect it'll mostly be England fans, but if you can, have a few drinks with some Russians. You won't remember much, but it'll seem fun at the time.
My mate and I are actually being hosted by a Russian guy and his wife, and they're taking us out in Moscow on Friday night, so should be plenty of mixing with the locals. To be honest going to semi try and avoid all the raucous and drunk England fans, bar the day of the game where I intend to have my top off and a pint of Fosters in my hand like the rest of them haha ;-)