Leiden Centraal is on the main rail line between Amsterdam and The Hague, with intercity trains running every ten minutes during the day. From Schiphol Airport the journey takes about 15 minutes and one direct train per quarter-hour serves the route — Leiden is often the most convenient base for a brief visit that involves the airport at either end.
From The Hague (10 minutes), Amsterdam (35 minutes), and Rotterdam (30 minutes), the train is far quicker and easier than the car. From Belgium or Germany, change at Rotterdam Centraal or Amsterdam Centraal. The old town starts a five-minute walk from the station.