Arlanda Express operates Stockholm Arlanda Airport to/from Stockholm Central Station every 10-15 minutes throughout the day. The travel time is 18 minutes. It costs extra to purchase a ticket onboard the train, so you are recommended to buy tickets online, at the airport information desks or from a ticket machine.
Commuter trains depart twice an hour from Arlanda and take 38 minutes to Stockholm and 18 minutes to Uppsala. Buy your ticket at the Arlanda Central station desk, in the SL app or at the normal SL and UL sales points.
Sweden's main train operator SJ also stop at Arlanda Central station.