I want to take a night train between Stockholm and Oslo in late July but cannot find any information on this route. Where do you go to purchase tickets and how far in advance can a booking be made?
I checked on the SJ (Swedish rail) website, and it looks like they don't offer night trains along this route. The latest train leaves Stockholm at 15:25 and arrives in Oslo at 21:24.
God, even Sweden's national rail website is infected with ABBA!
A very good place to look for schedules all over Europe is on the Bahn website, http://reiseauskunft.bahn.de/bin/query2.exe/en.
There is an "overnight connection", but it is not a night train. In fact it consist of a 3 hour train from Stockholm to Göteborg, a 7½ hour layover in Göteborg (from 23:25 to 6:55), then a 4 hour train to Oslo.
There are a number of daytime connections in under 6 hours.
I recall reading somewhere that SJ had stopped running the overnight train. In any case, the SJ website has the timetable posted and it doesn't show any night trains.
http://www.sj.se/sj/jsp/polopoly.jsp?d=7343&a=77257&l=sv
There has not been a night train on this route for several years. There are just a couple of daytime trains and flying is also competitively priced.