How to travel to the Brienzer Rothorn
You first need to travel to Brienz. There you will switch to the steam train.
The trip in detail
Brienz can be reached by car, by train (20 minutes from Interlaken, 90 minutes from Lucerne) or by boat from Interlaken. From the train station, you only need to cross the street to reach the steam train station. From the boat dock, it's about 2 minutes on foot to the steam train station.
See here for further details if you want to make the scenic round trip via Giswil and Sörenberg.
Check the timetable
This excursion can not be done all year round, so be sure to check the timetable as explained here.