Hi Noridaha, and welcome to MySwissAlps!
>> Basically I want to take golden pass train from lucerne to Interlaken OST because from there I want to go to Wengen but want to ride on golden pass train. Is it call lucerne-interlaken express?
Yes, the route from Luzern to Interlaken Ost is part of the Golden Pass route, and the train that runs along it is called the Luzern-Interlaken Express.
The full Golden Pass route runs from Luzern to Montreux, with a change of trains at Interlaken Ost, Zweisimmen, and sometimes Spiez.
>> I want to take golden pass train from lucerne to engelberg because i want to go to mount titlis. so the train is call lucerne-engelberg express?
Yes, the train is called the Luzern-Engelberg Express, but is not part of the Golden Pass route.
>> I will have STP. Do i have to pay any extras for these trains?
No, there are no extras to be paid.
>> What is the frequency, eg. every hour? I can just go to lucerne station and board the train?
Yes, you can just board the train with your Swiss Travel Pass.
You can check the frequency and departure times in the Swiss Timetable: