Trains are a good way - they leave hourly from Athens Larisa station to Korinthos' new train station and take about 80 mins each way

to cover 55 miles i'd hardly call these high-speed but it is a new line - there are a few daily IC trains Athens to the old Korinthos station that take about 10 minutes longer each way

There are no trains to the old Korinthos station. The new Proastikos trains run on the new standard-gauge line from Athens to Nea Korinthos and Kiato where passengers now have to change to the old narrow-gauge line to continue to Patra and stations in the western Peloponnese. Schedules are on www.ose.gr.
I don't know whether the new Korinthos station is near the ancient city.

I was quoting Thomas Cook 2007 timetable about IC trains to old Korinthos station - these obviously have stopped running

the old station is ten minutes or was 10 mins by train from the new according to Cooks 2007 which said IC trains Athens-Patras stopped at both

I did the journey in March 2007. Proastiakos trains ran from Athens to Nea Korinthos where you changed to the narrow gauge line for the old Korinthos station and stations beyond. There were no through trains from Athens to the old Korinthos station - that would have been impossible as the tracks were different gauges.
In the past year, the new line has been extended westwards to Kiato which is where you now change to the narrow-gauge Peloponnese trains. Eventually, the new standard gauge line will go all the way, so the timetables will continue to change.

Thanks everyone for the info. I am assuming that everyone is of the opinion that the train is better than the bus. Is that a correct assumption on my part? Thanks again for your help and happy travels to all.

Question to GeoffHamer:
Since you did this trip from Athens to Corinth in March, did you happen to go over to ancient Corinth? If so, what public transportation is available?

Not if the bus goes right to ancient Corinth??? bus can't be much slower than the new slow-speed train

To answer Museumlover's question, I can't help on ancient Corinth. I merely changed trains at Nea Korinthos en route to Diakopto.