While relatively few tourists took advantage of Greece's train system, it was a handy option. I enjoyed traveling from Northern Greece by train recently, taking the train from Katerini to Athens and enjoying the scenic views along the way. The "Friendship Train" running between Thessaloniki and Istanbul also attracted travelers between the two countries, and on the Turkish side ended at the Istanbul terminal made famous in the film version of "Murder on the Orient Express".
While many domestic routes have also been cut back, some of them remain in operation but check locally at the time of your trip. For the foreseeable future, until the schedules are solid again, don't rely on a train journey as part of your trip to Greece.
More from Bulgaria on the cessation of trains into and from Greece: Greece gives up its international trains over debts?