score:6
There is no need to book anything in advance here. Go to the station, buy a ticket and take the train. After checking at interrail.eu, it seems that the trains from Interlaken to Grindelwald and Lauterbrunnen are not included in the pass.
There is no cable car to the Jungfraujoch. You have to take a train from Grindelwald or Wengen to the Kleine Scheidegg station. From there you proceed by a second train to the so-called top of Europe. Both these trains are rack railways. Maybe the author of the "Rough Guide" was a bit imprecise on that ... Note that the Jungfrau railway is not included either in your Interrail pass.