John:
Good idea for a unique model that really makes sense!
Some suggestions:
The spikes would be in small wooden barrels (they are heavy): Put a (spike) colored filler in the barrel, then enough real spikes on top to cover the fact that it is a filler. Lots of spikes were used, so probably several barrels. All marked as containing spikes, but most unopened.
Joiners (“joint bars”) in the early days were simply rectangular pieces of bar stock wide enough to ‘fill’ the web of the rail. Typically ~24" long with 4 holes for the connection. They were soon modified to be ‘taller’ than the web space and were machined (I’ve actually seen some that appeared to be torch cut) to have some conformance to the rail foot and the rail head. Kind of a trapezoidal shape. Only relatively modern ones have the formed shapes the provide full contact with the rail web area.
Also, there would be barrels containing the bolts and nuts required for the above joint bars. Same situation: One open barrel along with several unopened, but all marked on the outside as to contents.
Be certain your track tools include the long pry bars for aligning the track structure, along with the fixed open end wrenches necessary to tighten the above bolts & nuts. How about some pipe cheater bars to provide more force on those open end wrenches?
Some pieces of indeterminate junk used for shimming, prying, whatever. Those cars were hard working and seldom fully maintained. What ever the crew put there was pretty permanent.
Several workmen would be on (or around) the car. All the things that workers require: Water jugs or bags would be a start. Some personal items such as lunch buckets and clothing items.
Just depends on how far you want go!!
Happy RRing,
Jerry