The high speed train station is located about 10km north from the Xi'an north gate (do not get mix up with the Xi'an North gate train station). The last train returns around 7pm. Schedules may change, so it is best to check it out at the station or website.
We took a taxi (RMB 32) from Xi'an city to the North train station. You could do so with public buses which required a bus change at WeiYang Lu (but not advisable as this is too troublesome and time consuming). We were there on a Sunday 8.20am and had no problem buying tickets for the next train at 9am. Obviously, you can buy tickets, in advance too. You can do so in Xi'an city, and we actually check this out with a street-side train ticket-booth (located on North street and near to the New Times Plaza) and the ticket-lady actually advised that there is no need to prebook, since we were not actually quite sure if we can wake up early enough and uncomfortable about the bus travelling time to the north train station.
The 2nd class air-conditioned seat costs RMB 188. The other seat type: 1st class costs RMB 288 (I would suggest you not be bothered with, as they looked almost the same to the 2nd class, except they are located in the 1st two cabins of the train and the seats are slightly thickened with more sponge).
The high speed train, being new, was clean and comfortable. The train stopped at several stations in between - and one of the stop was Huashan Mountain. Watch out for some breath taking scenery along the way, particularly around Huashan.
Once in Luoyang-Longmen station, there are many buses there to bring you to the "must-see" Longmen Grottoes (10 mins) and to the city (30mins). If you are there, and it's flowering time, do visit Luoyang Botanic gardens. We took a taxi from Longmen Grottoes (RMB 30) to the gardens and saw beautiful flowers, including their famous peony.
The fast train enabled us to take a one-day visit to Luoyang. We were back in Xi'an by 7pm. Without, we will have to put aside at least 12 hours of travelling time, which would imply we will need to stay overnight at Luoyang and be unnecessarily tired out.