The Hellas Express night train runs from Belgrade in Serbia to Thessaloniki in Greece. The full journey, via Skopje in Macedonia, will take you about 15.5 hours.
Although this is one of the fastest ways to travel to Greece, keep in mind that a large part of the route is not covered by your Eurail pass. Serbia can only be included in a Eurail Select Pass and Macedonia isn't included in any of the Eurail passes. This means that you will have to pay an additional fee for this night train.