>> Can anyone advise if the trains will be crowded on the long weekend of 17 & 18 Sept 2017 when a public holiday falls on the Mon 18th? I will avoid flying into Switzerland on 18th Sept if thats the case.
The trains will quite possibly be crowded, but usually only in 2nd class, and probably only for some routes and some departures. Closer to the time of travel, you can look up your intended trip in the SBB online timetable, and it will show how busy any given train is likely to be, both in 1st class and 2nd class. Personally I wouldn't change my plans for that reason.
>> Also are trains generally crowded on normal weekends as well? we will avoid travelling on a Sat and Sun if so.
On some routes they can be, especially in good weather, as lots of locals are out and about in addition to the tourists. Most people will be travelling 2nd class though. Again, check the timetable closer to your travel date.
>> we are planning to travel 1st Class on these routes (except mt trains). We have medium sized luggages with us (not using the forwading services):
Zurich Airport - Luzern - Engelberg (Mon 18 Sept or Tue 19 Sept) 2017
I would not expect this to be crowded, certainly not in 1st class.
>> Engelberg - Luzern - Wengen (23 or 24 Sept 2017)
In my experience last September, the train from Luzern to Interlaken was often crowded in 2nd class. Not so crowded that you couldn't get a seat, but a bit inconvenient if travelling with luggage. When I was travelling with luggage, I upgraded to 1st class, which had plenty of spare seats and room for luggage where I could keep an eye on it.
>> Murren - Grindelwald on a Sat 30 Sept 2017.
I would be surprised if it was crowded then. It is right towards the end of the season, when many hotels close until about the week before Christmas, so many people will have left.