By train. The slow but scenic train from Colombo Fort runs once a day to Polonnaruwa (about 6 hours), continuing on to Batticaloa. Book ahead: the second-class observation cars are popular. The station is at Kaduruwela, about 4 km east of the ancient city.
By bus. Express buses run frequently from Colombo (around 6 hours), Kandy (3-4 hours) and Dambulla (90 minutes). Local buses from Kaduruwela bus station serve Sigiriya and Habarana for onward connections.
By car. A hired car with driver is the easiest way to see Polonnaruwa as part of a Cultural Triangle circuit with Sigiriya, Dambulla and Anuradhapura. The roads are reasonable. Tuk-tuks for the day cost a few thousand rupees and are useful for hopping between sites within the ancient city if you don't fancy hiring a bicycle.