Mama has received a telegram - Uncle Chris is dying. She calls her three sisters and they all meet at a train station for the trip to his ranch in Northern California. She brings Katrin along; she is intimidated at the prospect of seeing someone die but tells herself that if she wants to be a writer she needs to experience everything.
Then ... The Sausalito sign can be seen at the end of the platform. This would indeed be where San Franciscans would have gone, by ferry, in 1910 to catch a train directly north...
... but this scene was filmed at the RKO Encino Ranch in the San Fernando Valley, pictured here in a 1947 photo. RKO's backlot ranch had three working depots; this one was referred to as the Brick Railroad Station. The RKO ranch was sold to developers in the 1950s and became the Encino Village subdivision (map).
Note that the train in the Then image above was pulled by Engine No. 29. This same one, or a renumbered contemporary, today provides scenic rides from Carson City to Virginia City in Nevada; the climb to 6,150 feet elevation guarantees spectacular vistas along the way (see it here in a promo video). Here it is in a 2010 photo.
The sisters, full of trepidation, arrive at the ranch house - Mama leads the way
Then ... It's a modest place, a little run down. This house and the barn in the background above were part of a working ranch, the Morrison Agoura Ranch in Agoura Hills 7 miles west of the RKO Encino Ranch. Many movies were filmed here before the ranch was broken up in the 1960s. The eastern portion has been developed into housing but the western part, where these buildings were, is still open land (map).
... in 1949 ... The scene below from the movie The Red Pony was filmed at this same house a year after the I Remember Mama scene was shot. These ranch buildings however are no longer there.
As Uncle Chris lies dying he reveals to Mama that his housekeeper Jessie is actually his wife - left unsaid over the years because it was not the thing to do amongst his society. He also admits that he has no money to pass on - she learns soon afterwards that it was because he had spent all he had on operations for crippled children including his niece Sigrid's young son Arne. His last request to Mama is that Jessie should get his house.