How can you buy train tickets at Oslo Airport?
In June I will have to travel from Oslo Gardermoen airport to Oslo Central Station to catch the train to Finse. I already booked a ticket from Oslo central station to Finse (with NSB), but didn't w...
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What you're looking for is the Airport Express Train (Flytoget). It's about 20 minutes ride from the airport to the Oslo Central station. No need to book a ticket in advance, there's always seats available. For timetable, check out http://www.flytoget.no/eng/Stations/Flytoget-Airport-Express-train-station-at-Oslo-Airport.

As for the ticket, you have two options. You can either buy your ticket from a vending machine that's located in the baggage claim area (somewhere near the exit/customs), or you can just swipe your credit card (and select destination) at the card readers that are located right before the escalators that take you to the platform. The card readers will not print a receipt/ticket. If you need a receipt, you have to register at the Flytoget site, and add your credit card info. You don't need to do this before you use your credit card. Once you register and add you card info, you'll see all the trips that were paid with your card, and you'll be able to print receipts.

I don't remember exactly how I managed to buy a train ticket at Oslo airport - so it has to be easy as I was there just recently. I remember that I bought it at a machine and paid by credit card.

An answer from tripadvisor forum (user dyoll, Destination Expert for Norway + Oslo, 04 June 2013):

"Ticket machines have an English option. However if you are looking at taking the local train from Gardermoen to Oslo then I suggest you just buy a Ruter# ticket from the Narvesen kiosk right after customs - Local trains are part of the Oslo transport network and tickets from Ruter# or NSB are interchangeable (and much simpler dealing with a human than those machines!)."

Oslo airport have the easiest ticket machines imaginable. You will find some turnstiles, leading to the train platform. They have a built in card slot, so you simply slide your credit/debitcard through a slot, and you are let through. Your card then becomes your ticket.

By default it buys you one standard ticket from OSL to Oslo S (Oslo central station), or whereever you exit the train – remember to slide your card at your arrival station.

If you need a discount ticket, for example student or elderly, you need to buy a ticket. Normal ticket machines can be located close to the aforementioned turnstiles. They are huge and orange, hard to miss.

By the way, the train service from the airport is not NSB, but Flytoget. Their website is http://flytoget.no/

Note, that while all stations have the card slots, not all platforms are closed off by turnstiles. This means, that you can enter the train to the airport, without sliding your card. When you exit the train at OSL, and go through the turnstiles, you will slide your card to be let through – but since the machine does not know which station you departed from, it will assume you travelled the entire route from Drammen.

There are actually several viable options, the cheapest would have been to buy a through ticket from the airport to Finse.

The most convenient option, but also the most expensive, is to take the Flytoget (Airport Express Train). Mo-Fr, the trains depart every ten minutes (x:00, x:10, x:20, etc), Sa+Su every twenty minutes (x:10, x:30, x:50) and take 19 or 22 minutes. They are however quite expensive with 170 NOK or about 21 Euros for a one-way ticket to the central station. You can travel without a paper ticket, which sounds quite simple, but you have to be careful not to charge yourself for a longer distance than actually travelled. Travelling from the airport is quite simple, as you must select your destination when swiping your card at the platform gate. If you travel to the airport, you must remember to swipe your card at one of the devices at the departing station, otherwise you will be charged 250 NOK (the tariff for the longest distance). There are also plenty of ticket machines where you can buy regular paper tickets and pay with cash or most credit and debit cards. If you want to pay with cash, be aware that the ticket machines won't accept larger bank notes than 200 NOK.

At least Mo-Fr, it is also a reasonable option to use the regular suburban trains instead of the express trains. They leave twice an hour (0:03 and 0:43) and take 23 minutes to the central station. These trains operate on Sa+Su as well, but not necessarily every hour. The ticket price is 90 NOK or about 11 Euros. You can buy tickets from the NSB/Ruter ticket machines or from the NSB ticket desk. If you are in a hurry, you can also buy your ticket on the train from the conductor, but in that case, have to pay an additional 40 NOK fee.

Be aware that the express and suburban trains are operated by different companies and their tickets are not interchangeable. Express train tickets are available at the orange ticket machines. Suburban train tickets are available at the red and gray ticket machines.

There are also several bus lines operating from the airport, but if you are going to the central station, the bus lines are neither a cheaper nor faster option than the suburban trains.

This is directly copied from this site - http://www.eurail.com/trains-europe/trains-country/trains-norway , which may have more information that you would find beneficial. The parts I have put in bold are the parts that I believe should answer your question.

How to make reservations for trains in Norway: - You can make reservations for the domestic and international NSB Regiontog day and night trains, as well as the international InterCity:

At a ticket desk or self-service machine at local train stations. This is the easiest way to get your tickets and the English-speaking NSB staff can help you with any issues.

By phone through the NSB call center (+47 815 00 888).

The NSB call center has English speaking staff (select ‘9’ in the menu) and is open on weekdays from 8 AM to 8 PM and on weekends from 9 AM to 3 PM. You can pay by credit card or pay cash when you collect the reservations at a ticket machine at one of the train stations in Norway. When booking through the call center, you have to pick up the reservations at least 7 days before the departure of your train. You can book other Scandinavian trains by calling the NSB call center. There is a booking fee here of 50 NOK (6.50 euros). At ticket offices in the larger train stations, you can book trains for other European countries.

I do understand you want flexibility and wanted to book when you arrive, but I thought I would put the other info on the phone line booking in case you wanted to call and ask them about more info as well.