You can find train timetables here: https://www.hellenictrain.gr/en

Not sure what the best source for bus timetables is.

Having passed by Denmark last week, on DSB (and a vandalised S-bane too): in fact about the only remaining TRAIN (tog)line in GR is the modernsied nord-syd Athinai-Thessaloniki, with some short bracnhes. This is not where most tipical tourists want to go. It thus also means there is NO train out of GR onward-shortest-payable-buslink is Thessalonik-to border BUlgaria (bus will mostly run onward to Sofia)

Of course most appealing are the islands, which will have no toget-bare faerje. There is also a Greek Ferry Pass-hard to get underway and not very useful either.

The green BUS reigns for most normal transit in GR-its all by KTEL -a coöperative per province, and a google will usually leads to results: ´Ktel bus from X, or ON island Y). For touristy areas the results are often better as for non such. The well-known buslinks as Flix etc are only allowed to offer INTERNational. Your Hotel etc-or the taxidriver to the busstation (every minor town will have it) will also usually know about onward transportations-or that old faithful; :Lonely Planet! switching/skyfte fra toget till bussen is however often fraught with problems and no connections

Here the Tutorial of @EdM how to book reservations online for Greece