3 years ago, I went on the same route: taking train from Jakarta to Rangkas, then to Merak port. However, i couldnt figure out how to buy a ferry ticket without local number so I turned back with regret. Here is the trip report 3 years ago
This year, I booked a bus from Jakarta directly to Bandar Lampung (Sumatra). The bus took 17 hours (10am to 3am) to finish the route due to picking up and dropping off passengers door to door. It is the month of Ramadan so most of us were fasting for the entire trip. Extremely tiring, but extremely exciting.
The ferry was an amazing experience, with the stars and nightly sea breeze. Definitely would come back again!