I can now confirm that after travelling on this train, it runs every day, according to the departure screen in Yerevan station, and tickets are easily bought at the station, although it can be difficult without some Russian. I was lucky enough to have a woman who spoke some English, and knew someone else who helped translate for me. It's not too expensive, less than £30 single for a bed in a 4 bed compartment. You will get an exit stamp from Armenian officials, and an entry stamp by Georgian officials midway through at the border. I traveled in February, so it was a very quiet train. Not a single other English speaker was on board! It's a truly fascinating experience!