Sort of depends on which train you're referring to. If you're referring to Arlanda Express, it takes 18 min to get you to ARN (barring an accident) and at that time on weekdays it runs every 10 minutes. ARN terminal 2 is small so 1.5 h is definitely sufficient time. You'll have fast track through security although T2 security usually doesn't take too long even without. If you visit the lounge "upstairs" (Pearl T2), just remember that you need to pass through passport control to reach your gate, i.e., after your lounge visit. Most likely, your flight will be the only non-Schengen flight from T2 around that time, so if there are queues, you're all heading for the same flight.