Precursor
Arlanda-Stockholm runs throughout the entire day, it's Stockholm-Skövde where the last train is at 18:something. So that's kinda weird.
50 minutes from landing to Stockholm is... probably impossible. The train only takes 20 minutes, but that means you have 30 minutes to make it from the plane to the station when you land. I wouldn't even consider it if you have any checked in luggage to wait for and collect, and even if you don't, it's probably pushing it. The flight bus from Arlanda to Stockholm C takes 45 minutes so yeah not happening.
50 minutes from landing to Stockholm is... probably impossible. The train only takes 20 minutes, but that means you have 30 minutes to make it from the plane to the station when you land. I wouldn't even consider it if you have any checked in luggage to wait for and collect, and even if you don't, it's probably pushing it. The flight bus from Arlanda to Stockholm C takes 45 minutes so yeah not happening.