I have been assuming that this structure would be similar to the Visitor Centers that NPS puts at various historical locations. Based on that analogy, I would expect it to contain the following:
(1) A theater showing a 15-20 minute film. Much of the show would be historical footage. The last few minutes would be of the current ride (to "hook" visitors to ride it themselves)
(2) A display of track gauge - what it is, how did "standard gauge" come to be the standard, and some information about narrow gauge. There might also be a display of wheel-and-axle sets to show the comparison of SG, 3' and 2' gauges.
(3) A display of how the area was settled by humankind. Included in this would be the various waves of Europeans, and why Spanish are different from Mexicans.
(4) A display of the rise and fall of the Rio Grande.
(5) A display of the development of CATS
(6) Perhaps a physical model, with NS/EW dimensions compressed to dramatize elevation shifts, of CATS, to show what the railroad has to contend with.
Each display would take up a "room" (physical or created through the use of dividers), so these alone would take up more space that would be available in the section house. When you add in various other functionality (to support researchers, for example), the current design is just about as small as they could get by with.
I have a meeting in a few minutes, so I'll have to end it here.