Hi Kanika, and welcome to MySwissAlps!
>> I am planning to visit Switzerland in mid-September. Can I catch the chocolate train if I leave early morning from Interlaken for Montreux? If yes which train and time would suit me?
In September, the Chocolate Train departs Montreux on Mondays, Wednesdays and Thursdays at 8:57. In order to get there in time from Interlaken, you would need to leave at 6:00 and take the fastest route via Spiez and Visp. You wouldn't be able to get there on time using the Golden Pass route via Zweisimmen.
>> And the station for the chocolate train is same where I will get down while coming from Interlaken?
>> Are there any delays in train usually?
>> Also once I will complete my Chocolate train and come back in Montreux around 6 pm, Can I take a golden pass route and take a panoramic train back to Interlaken? Is it possible?
Yes, it's possible, but would take at least 4 hours. Sunset will be around 19:40, so a lot of your trip would be in the dark, and you would of course miss the scenery.
>> If not please advise other feasible options.
The fastest way back from Montreux to Interlaken via Visp and Spiez takes around 2.25 to 2.5 hours.
If you just want to go to the chocolate factory without travelling on the Chocolate Train, you can get there on local trains via Bern and Bulle in just under 2.5 hours. The stop you need is Broc-Fabrique. Going via part of the Golden Pass route would take at least 3.25 hours (Interlaken Ost - Zweisimmen - Montbovon - Broc-Fabrique)
You can use the Swiss Timetable to plan your trips: