I'm definitely interested.  I

I'm definitely interested.  I use a 3d printer to print stuff for a station I am building.

You did mean 3 AXLE... No?

(I Must admit, 3 trucks would be an interesting animal!)

Maybe Walthers trucks?

I would suggest using Walthers heavyweight 6-wheel trucks for this purpose, if you can round some up.  They are made of metal, so they have the weight down low where needed, and they also pick up power, so lighting could be added as desired.  Walthers offers a few different varieties, so each car could be different, or you could standardize on 1 style. 

https://www.walthers.com/commonwealth-straight-equalized-trucks-w-outside-bearing-arch-roller-bearings-1-pair

Gives a place to start.

The 4 screws that hold the truck together could probably be repurposed to hold the body on, and conduct power.

@Walthers Trucks and typo

Fixed the typo and as to using the walthers trucks I'd try that if I had them on hand or some money to get them spent this rounds hobby budget on my other interest coin and money collecting so will have to wait a bit to pick those up. 

An update on the engine for these cars I took my old AHM BL2 I had which had a seperate powered truck with a decent motor I chopped the shell in half to fit on each truck individually the next and final step now is just to cut backs for the shells and find a good way to blend them in I will probably eventually design a specific 3d printed engine but it will do for now.

S Scale???

Are you doing the plans at full scale or model scale?  Can I get the plans into other scales?

Great idea!

I will buy one of each; let me know when ready.

The metal trucks are a good

The metal trucks are a good idea... In N Scale, I had to use three 1/4-20 nuts in each car to get them to run right. Plastic 3D prints don't have much weight to them!

Scale

I am drawing them specifically for HO trucks if you have dimensions of the frame you are using for S scale I could scale them up

Selling

The files will be avaliable for download free soon I am wrapping up the body file and the roof will be a seperate piece as for selling we could possibly work something out as fundraising for our engineering club since that is where I have the printers all of the sale would go to the club.

First Print I think the stairs could be a bit lower