Propane and propane products. LPG tanks are found all over rural communities , and affordable Z scale tanks would be a nice addition to Z scale, to enhance your farm and country houses. These are also great to have in small towns too.
These could be made easily using a lathe and them molded, or using rapid prototyping. Silver or white would be great colors to have them in.
I am attaching a link I found of a company that makes the full sized ones. They have pictures of different size tanks and also the dimensions.
If these could be made cheap enough, it might be nice to actually have a propane dealer that sells them. Of course you would need the other size bottles and a BIG tank along the railroad to go with it.
www.missiongas.com/250gallontank.htm 
