Well Joe, your answer seems to verify the others'. I've been there a few times myself, and I am a known "train nut" so I enjoy the visits, BUT I can see the general public's frustration with the museum. There are few, if any interactive displays for the general public. Yes, you can walk through the train sheds and look at many different cars--BUT to the uninitiated (in other words, the average visitor) they all start to look alike, and I haven't noticed any docents telling the stories of the various piece, which MIGHT make it much more interesting.
Being a former museum professional (IE: I was a PAID Curator), I understand the need for a museum to have a mission and to stick to it. However, when the mission becomes so narrow that it negates pubic support, then it needs to be re-evaluated. Rio Vista is in an interesting area, somewhat removed from all the development around it, yet close enough to provide great demographics for high visitation. The museum right of way south is a great reconstruction of the electric railway system. The north side COULD be a great example of non-electric railroading, utilizing artifacts already in the collection. Such an expansion of the mission would also attract a wider group of volunteers, some with the skills necessary to maintain that type of equipment.
You mention that PLA runs steam & diesel and that the two museums complement each other. Well, yes they do--at least academically; but they are not close enough to each other to be a "complementary" experience to a one-day visitor.
The entire museum community needs to pay close attention to their audience and the general public's perception of them; their long-term survival requires it.