Of course it's staged! It's a TV show. They don't have days to wait for something to happen. As they explained in the "after-show" segment where they answer various questions, they were pretty sure the real tank car would implode after being thoroughly steamed and sealed with any relief valves plugged, based on prior testing. But it was hot outside, and things weren't happening fast enough even with water being sprayed on the car for cooling. They calculated what vacuum should be generated, then did it again with a pump (oh yes, and with dropping a big hunk of concrete on the car to make a dent for the implosion to start at) so they could get something to show on TV before the crew went on overtime. It's entertainment, folks, with a little science thrown in. It's common for Mythbusters to "help things along" when needed for the effect.

Of course, being in eastern Oregon also meant being at a good altitude (at least a couple K feet), which means the atmospheric pressure would be lower than it would be in their "lab" near sea level. IIRC somebody said that they often worked in the Camp Parks area near Livermore. Wonder if they forgot to put that into their calculator?

I do wonder what was done with the car afterward. Presumably it was going to be scrapped anyway, but it certainly wasn't road-worthy after that treatment. Must have been scrapped in place, wherever the "place" was (in eastern Oregon per the after-show). Cutting them up with a portable (on a truck or 2) plasma torch (closest thing we have to a real light saber) would have been another good show segment.