How to book the mongolian train from Bejjing to Oulaanbaatar in Mongolia ?

HI there, I would like to travel from Bejjing to Oulaanbaatar on 6th of february 2024. I was looking to find train tickets online but coulnd't find it. Any chance you can direct me or are you managing bookings as well ? Are trains still going to Mongolia from Bejjing ? (as I found some informations that it's been paused due to covid, but Covid is gone now, isn't it ?) thank you so much for your help.