Greek rail travel is an adventure if the one and only time I used my FIP coupon a few years ago is anything to go by. Having arrived in Istanbul on the ferry from Odessa, the planned journey over the course of a few days was Istanbul - Thessaloniki (sleeper) - Athens - Patras (for ferry to Bari)
1) Train from Istanbul to Thessaloniki was terminated at 02.30 in the middle of nowhere as the line ahead was blocked. An earlier train had hit a lorry on a crossing, a serious incident involving four fatalities. So chucked out of bed and onto a replacement coach to Thessaloniki.
2) Train from Thessaloniki to Athens was terminated a mile short of Athens, due to a political demonstration blocking a level crossing, we all had to grab our bags and walk the last mile along the track, past the demonstration, with the police just watching on.
3) Train from Athens to Patras involved a change at Corinth, something we didn't realise at the time. Only worked it out when we saw the connection departing. Result - an unexpected two hours in Corinth.
So good luck, don't let me put you off