Going to the Maijishan Grottoes near Tianshui is a nice day trip from Xi'an using the high speed train line. Here are the steps for getting to the grottoes from Xi'an.
1) Take the Xi'an Subway Line 2 to the Xi'an North Train Station (the last stop going North).
2) Xi'an North ticket lines are very long. Advance tickets can be purchased online, for example on Trip.com (formerly Ctrip), but they still have to be picked up at the ticket counter. Allow 15 to 45 minutes for waiting in line.
3) The fast train arrives at the Tianshui South Railway Station which is on the edge of the city. There is a little bus station right in front of the train station. Bus 60 goes directly to and from Maijishan, which is about 30 kms away. Currently, there is a massive road construction project for about half of the trip so the bus takes about an hour each way.
Option 2: It's also possible to take a taxi from the Train Station for about 200 CNY roundtrip if the driver waits or about 120 CNY each way.
4) At Maijisan, there is an optional electric tram that goes the from the entrance to the mountain. It's a 3 km to the Grotto site so the tram is recommended to save time.