According to a Norwegian newspaper it will soon be possible to check in at Oslo central station in downtown Oslo before you take the airport express train to Oslo airport (OSL) – if you are flying with Norwegian or SAS that is. But it does not seem like they are going to go all out on this…you will only be able to check in and then you have to bring your luggage to OSL and go to the baggage drop. In Kuala Lumpur in Malaysia you can also check in at the train station (KL Sentral) before taking the airport express train to KLIA but there you actually check in and drop your baggage which I find to be an excellent solution. But hopefully checking in at the train station in Oslo will mean shorter lines when you do get to the airport. Here is the article (only in Norwegian).