Which pass to get?

You get to Mürren with your luggage via 2 different options from Lauterbrunnen
1. Cable car to Grutschalp + Train to Mürren
2. Bus to Stechelberg + Cable car to Mürren

They have self luggage check sometimes available down in Lauterbrunnen for the Grutschalp cable car which will transfer your luggage to Mürren, but they don’t advertise it at all and you have to know to ask the people working there. If they allow it, they’ll open a rope to the side for you to load onto their cargo hold below the cable car and it will be on the back of the train on a trailer that’s attached in Mürren for you to retrieve. This service is free, you just have to know to ask. If they don’t allow you or you don’t want to, you just carry your luggage yourself onto the cable car and train.

For the other route, you take your luggage on the bus and they offer a paid “check” of your luggage on the new cable car.

For passes, look into the half fare card + Jungfrau Pass.

Just how much luggage will you have?

If we just have carry on luggage, can you walk from train station to Murren? How long would it be and how steep?

What do you mean? The train station is in Mürren, so you’re already there when you get off the train. From there, a walk to your hotel depends on which one you’re staying at. Do you have that booked?

Depending on where your hotel in Muerren is located as to the best way. The train station is on the north side of town, and the cable car station is on the south side of town, so one or the other may be the shortest walk to your hotel. Either 1. or 2. as described by wanderweg.