Which is the direction to travel between these two cities for the views from the train in early September?

Doesn't really matter at that time of year - there is still sufficient daylight. However the section between Geilo and Oslo is not particularly interesting and as it's a long day I would choose Bergen - Oslo direction then you can safely snooze from Geilo to Oslo without fear of missing anything. Seen plenty of jetlagged tourists nodding off on the interesting bits!