Is it possible to visit the Datong Yungang Grottoes on a day trip from Beijing?

Well, I'm afraid you don't have enough time to visit the grotto within one day because it will be closed at 18:00 pm. You are advised to go to there one day in advance to enjoy the beautiful scenery. After your sightseeing, you can can take Express Tourism Bus Line 603 for about 1 hour to reach DT train station, and the ticket fare is CNY3/person. Then, catch train to Beijing for about 6 hours and 30 minutes. Ticket fare of soft sleeper is CNY152.5/158.5, hard sleeper is CNY101.5 and hard seat is CNY47.5/53.5.