My take is that most of these things can be valuable, in the right context.
Eventually, in Portola, we want to build a visitors center and display area. There we will talk about the technology, the historical context, etc. In that case, things like spike mauls, trainorders, tie plates, brakes shows, etc. become important because they give a physical presence and context to the history. We have plans to take some old trucks and a tank car frame, seemingly useless, and make an interactive display on air brakes and wheel bearings.
So, this is a long-winded way of saying the cop-out answer that all of it is possibly valuable in the right context. It depends what the museum is aiming for in terms of display and interpretation. SOmething can make a great part of a display if nothing else. But you don't know that until you start making the plans and designs for the display.