24h direct transit, train from Russia to Beijing

I've read that if you travel to or from Hong Kong by the through train, the time in Beijing is calculated from the time you step on or off the train in Beijing, not when you enter or exit China.

But is it with the Trans-Siberian trains coming from or going to Russia and Mongolia? Are the 24 hours calculated from the border or 12 hours later when you step off the train in Beijing? If the latter, then I guess one would have time to see the city and fly away within 24 hours.