For the eight train cars in the puzzle I wanted to keep the cars as short as possible to keep the layout small. This is just a puzzle, not a 'normal' layout. It will be unplugged and set aside so valuable floor space can be used for other things. 

There needs to be some way to label the eight train cars. Open gondolas or ore cars looked like a place to start to have some place to carry the label.

I did fined a prototype 22' ore car kit. After putting one together, I questioned if the plastic wheels and trucks were going to be reliable. I also felt I would need to replace the couplers to reliable couple/uncouple cars under computer control.

Next in size up was 26' ore cars. Found these in sets of four. For a few dollars more I also found similar gondolas with eight different rail-line/paint schemes. Eight different gondolas has visual appeal, but randomizing an answer, displaying a target answer, etc., all become complicated. So I was thinking I would end up numbering them somehow anyway so went with the two sets of four option.

The internet revealed several option for labels. Someone used tiles from the Scrabble board game to label their N scale gondolas and flat cars. Kid's lettered blocks or domino tiles were also used.

Gondola with chil's numbered block at an angle Gondola with child's block inside bed Gondola with different child's block at an angle. Gondola with a domino as number label Gondola with Post-it note for a label

If I was artistic or a painter numbering the cars with 'graffiti' sounds better than plain colors like the laptop graphics or just simple numbers. Post-it notes doesn't sound like a very good idea. Sense I have the tools, for me simple "engraved" numbers seemed to be the best answer.

With some modification to match the sloped insides of the ore car, the blank shown below is where we are headed. The top is sloped and provides a 1" x ½" pallet with a border.

Ore Car with a blank label

If I am going to make eight of these labels, I need a jig to hold the aluminum blank.