Is Golden Pass Express reservation necessary in mid/late June travel?

1. All 3 legs show "reservation possible", with the Zweisimmen-Montreux leg showing "reservation recommended". Cannot say how crowded will be in June.
2. The Luzern-Interlaken section, and the Zweisimmen-Montreux section have panorama cars. I do not believe the Interlaken-Zweisimmen does.
3. Interlaken Ost station is in a commercial district of Interlaken, so no worries there. It is a 13 minute connection, ages by Swiss standards. I'm seeing you arrive on track 4, and you will need to use the passenger tunnel to get to track 8 and go up the stairs to the platform. I have not been to Zweisimmen, but I do not believe you will miss a connection. The 2 trains are designed to meet one another, since you are going from standard gauge track to narrow gauge track. If the waiting train left without connecting with the incoming train, it would probably leave empty.
4. Should be plenty of space for your luggage. Overhead racks, and space at the ends of the carriages for large items.

This route, like all other rail routes in Switzerland, has one train per hour throughout the day. You can travel on any train, and see the same mountains, but on other trains you just get normal sized windows. It is ONLY the "Golden Pass Express" which has reservations, on the other trains there are no reservations.

For what it's worth: 15 years ago we took the GOlden Pass from Interlaken to Montreux and the train was packed in June. Luckily we had seat reservations and the people in our seat looked quite disheartened when they were required to vacate. Point of the story: If you absolutely want a seat then book a reservation.

If it is crowded, and you all want to sit together, it might be a good idea. I'm sure whether or not it is a weekend matters. You could board the train early in Luzern and find seats together, but it would be every man for himself when you have to make the first change at Interlaken Ost. A good percentage of the passengers will also be making the change, along with people who are starting the journey at Interlaken. You might not get seated together then without a reservation. Zweisimmen could be a real scrum on a busy day as virtually all the passengers will be making the change.

Seat reservations are 5 CHF per seat, per individual train, 3 separate trains. Maybe you could save the fee on the first leg, as if you get to the station early, the train will be sitting in the station for 10 minutes and you can board early and get seats. Provided, of course, that some else has not reserved them. But the other legs it might be prudent to pay the reservation fee.

Sam, Chris, and Tim:

Thanks a lot for taking time to reply and provide your sights!

Best regards!

FJ