Originally Posted by northinsouth

So, they're absolutely reliable but it's variable in terms comfort and time. There is an express route that goes from Tbilisi to Batumi with a stop at Kutaisi International Airport and (I think) Poti on the Black Sea. Those trains are really nice. They're fast and efficient and they can take you from one side of the country to the other in about 6 hours. Kutaisi is the mid-point. In winter, it's 2x per day, but I think in summer it's closer 4x per day.

There are other local options, such as going to Zugidi (which is basically at the foot of Svaneti where you should absolutely go if you have time), but those trains are older and less comfortable--but also cheap. Georgia Railways should have all schedules online, though be aware you can only buy tickets roughly 3-4 weeks out from date of travel.

The one exception to the train is the Eastern side of the country, particularly Kakheti. Unfortunately, you'll need to either rent your own car (driving in Georgia is a sport unto itself) or hire a driver in Tbilisi. Alternately, you can take a marshrutka, but they're not exactly the most comfortable forms of transport.

Hope this helps. Let me know if you have other questions.