You bring up a very good point, and one I have complained about for decades. Locomotives get saved and displayed, but what about the lowly boxcar, flatcar, tankcar, etc? These cars created the need for the locomotive. The locomotive was just a way to propel the merchandise-carrying equipment. Museums need to display a TRAIN, not just a locomotive.
Of course, it's hard to get folks excited about fixing up an old metal (or wood) box; they just aren't as "sexy" as something big that moves itself (and makes appropriate noised while doing it).
A few museums get it, but not all of them do!