Hi Navin, and welcome to MySwissAlps!
>> I am planning to visit Jungfraujoch from Lucerne on 29th April 2018 ... It shows that it will be snowing on that day. Is it still worth visiting the top?
Not if it is snowing, as you won't be able to see anything. However, the weather forecast is not reliable more than 24-48 hours in advance, so check again closer to the day.
To check visibility on top of the Jungfraujoch, use the webcam:
>> I have bought a swiss rail pass. So, what are the traveling options from Lucerne? And which of them should I reserve ahead of time?
No part of this trip needs to be reserved.
The route to the Jungfraujoch is described here:
www.myswissalps.com/ju ngfraujoch/gettingther e
You can use the Swiss Timetable to plan your trip:
So you first get a train from Luzern/Lucerne to Interlaken Ost, then proceed to Kleine Scheidegg and the Jungfraujoch via either Grindelwald or Lauterbrunnen and Wengen.
With a Swiss Travel Pass, you will travel free as far as Grindelwald or Wengen, and get a 25% discount for the rest of the trip to the Jungfraujoch. You can buy your tickets for the trip between Grindelwald or Wengen and the Jungfraujoch (and return) at any staffed railway station in Switzerland. However, don't buy them before you have confirmed the view on top is clear: you could do this at Interlaken Ost, Lauterbrunnen or Grindelwald stations.
There is some construction work on a section of the railway line between Luzern and Interlaken until 29 April 2018. Replacement buses will be provided between Horw and Giswil. It will be well organised, and nothing to be concerned about.
>> I am also planning to take a trip to Mount Titlis on the 28th of April. I understand that it is covered via swiss pass.
You get free travel to Engelberg, then 50% discount on the gondola to Titlis from Engelberg. The route is described here:
Check the weather and visibility before going up:
>> Do I have to make any reservations for this?
No, reservations are not possible.
>> Also, is there a better option than the trip to Mount Titlis?
Opinions will vary. All the mountains near Luzern offer wonderful views in clear weather, and none would disappoint. Read the information, look at the photos, and decide what's right for you.