For Mount Titlis, you do need a ticket from Engelberg; you do get a reduction of 25%; the reduced price for Engelberg to Mount Titlis is 72 CHF return per person;
For Jungfraujoch, you do need a ticket from Grindelwald or Wengen to Jungfraujoch (possible to buy online but I would not do this long before and would have a look at the weather forecast before); you do get a reduction with your Eurail pass here, too. The reduced price is 148,60 CHF per person return from Grindelwald or 150,20 CHF per person return from Wengen; seat reservation (not mandatory) is 10 CHF additional.
The train from Interlaken to Grindelwald and the train from Interlaken to Lauterbrunnen and the one from Lauterbrunnen to Wengen is included.