Not so sure...
Unless this has changed recently I cannot confirm that it's that easy...
Last year (2001) we entered Mongolia at this bordercrossing north of UB, and we had to take thetrain! Because of our big and heavy bikes it was not possible to take them in the train, and we had to arrange some Mongolians to have them pushing our bikes across the road border. But with your 400 bike, you should definetly try to take it in the train.
This whole hassle took us two days, both or entering and exiting this country. Anyway, it's worth it!
Maybe Vincent has been there more recently, and the regulations have changed. When we were there in August 2001 they were just in the process of constructing a new (appearently international) crossing, but it didn't seem to be even nearly finished. Anybody have some reliable infos about that?