<<"i would add that the weather here is much better than the forecast.">>
I find that a forecast of rain is almost always true, but not very precise or fine tuned. Often, an area will have rain in one part - which must be reported because it it there - but not in other parts, and so I often find the same result as you do. Unless the forecast is really, really awful, I wear light rain gear, and go anyway.
The waeter forecast on Swiss TV ( SF1) in the early morning hours is pretty good, and makes some attempt to show details in various regions. Look very carefully at the icons; there can be subtle differences. In between periodically repeated forecast maps, there are webcams of several towns or (in the winter) ski slopes.
Not too useful for fine details in the Jungfrau region, however.
The one place where accuracy is critical is if there are clouds up on the Jungfrau. It costs a lot o get up there, and it could be an expensive way to see the inside of clouds. Ask at local train stations, touristinformation offices which may have webcams, your hotel reception.
<<"The space between cars [often at the ends of cars - Slowpoke] is limited if there are more than a few passengers, the aisles are narrow for moving luggage, it's tough hefting luggage on and off the trains, and in some cases we had to walk up steep stairs between trains hauling our big bags.">>
If I have big bags, and expect to stay in one place for more than a day or two, I always use the SBB luggage shipping service for my large bag/bags.. It helps keep my back healthy, at a modest price for good health.
There are different services which have different speeds, and I take the slowest one, which costs 12 CHF per bag and a couple of days to get there. Many stations, including some in the Jungfrau region, also have faster services at higher cost.. Delivery to a hotel is possible for an extra fee in some places.
I pack a small bag with supplies for a day or so, and carry it.
If you go from hotel to hotel frequently, those costs can really add up for a family. Or, sore backs can add up, too. That is one reason we often recommend not more than few bases for a longer trip.
I have rather large feet and if when I pack my hiking boots, they take a lot of space, and my hiking staff likes to be inside a long bag/large bag. So, I use the service whenever it fits my schedule.
Main stations have good ways to handle large bags, but as you get out into the mountains, on small trains on meter gauge tracks, and at older and smaller stations that do not always have elevators ( lifts) big bags can really be a problem. I recall rather narrow stairs and tight connections at Interlaken Ost.
Lauterbrunnen does have elevators. Wengen is flat.
If you have large bags, make sure they have wheels.
I pack a small bag with supplies for a day or so, and carry it, and ship my large one. For long trips, I have learned which hotels/inns can wash a few clothes for a modest price, or have D.I.Y facilities (common in many countries.). (Not at hotels in big cities....expensive...).
Do it yourself laundry facilities exist, but they are not common. The washers can be small and slow and in steady use, such as the one in Wengen near the Hotel Silberhorn. I recall that there is one in Lauterbrunnen, somewhere. Most Swiss have their own, or have shared facilities in their apartment buildings, so there are not many D.I.Y services. Tourist towns like Interlaken will have a few There are usually facilities near campgrounds, but those are often a bit distant from a train station or town. There is usually one near a university, as in Zürich.
The lack of easy to find D.I.Y laundry facilities means that I tend to pack more clothes than I would for a trip to many other places on a two or three week trip.
Rental apartments often have laundry facilities in the building.