When are you traveling? Timetables up date on 15.12.2015 and there may be minor diffrences after that date. So, you should use your exact date.
It gets dark at around 1630 0r 1700 in December....can't see the scenery.
Here are some details.
Arno and Annika have those details memorized. I am going by generalities.
This is always true -
You do not need a reservation to ride any regular train on that route or any route solely inside Switzerland. International routes ---such as to Italy - require reservations once you are over the border, so it is good to get them for the Swiss part, too.
If I recall correctly, you only need reservations for the panoramic or VIP cars on a Golden Pass train. Please check the link above. I think that other special scenic trains may require reservations if the whole train is special, such as the Glacier Express.
If you miss the train with reserved seats you can catch the next train, without reserved seats.
Te views are great, even in a regular car. The switchbacks above Montreux let you see the sights from both sides of the train.
Here is some useful information, including information on trains and tickets:
Trains are free with the Swiss Travel Pass, as long as they are public transport, usually meaning between population centers.
Special excursion and privately owned trains, such as those up to Jungfrau, must be paid extra for, but there is usually a discount. Thus, on the route from Lauterbrunnen to the Jungfrau, the section as far as Wengen has no extra charge. After that, there is an added cost.
Both kinds of trains are in the timetable, as are cable cars, boats.....
You may bring your own food and drink. Please be neat with the residues. It is a custom in Switzerland. ;-)