In reply to dave frost:
Here is a website with timetable:
http://www.nor-way.no/?lang=en_GB
Enter your travel date and enter departure from Oslo Lufthavn (the airport) and Rjukan rb.st. as destination.
The trip takes appr. 3 1/2 hour. The first part by train (NSB) from the airport to Kongsberg, and then a connecting bus further to Rjukan.
The whole trip costs 454 NOK, appr 45 £.