We managed to ride most of the metre-gauge system around the Peloponnese in 2003 (all by DMU though - missed out on being hauled by one of the old Alco's

). Great pity there is not much left in use, but away from the coastal areas it's pretty hilly, making the rail routes in those areas twisty and slow - I think express buses could easily beat the train schedules.
It's a great part of the world to explore though - away from the big towns I remember it being pretty unspoilt and very scenic. If you are interested in ancient civilisations and ruins,
Mystras (a Unesco World Heritage site) near Sparta is really interesting to wander around - amongst other things the remains of a Byzantine town are there.
From what I remember, the main rail centres (in terms of having rolling stock stabled there etc.) were Corinth, Patras, Pyrgos and Kalamata, so those are the places I'd head for if you want to see what's left (even if it isn't moving).