@Eccentric Crank posted:Anybody ever think realistically about their requests?
Some do......well, maybe that's optimistic, 
But it seems fewer care to be connected to reality.
Like can 3rd Rail make money by doing them?
Possibly....
Did enough railroads have them?
Sometimes it only takes 1
Variety of roadnames and paint schemes?
Sometimes it only takes 1
Popularity amongst model railroaders?
Very subjective popularity is....
Length of running era?
Sometimes it
Sometimes that's not a big issue, but one concedes that it can help.
Been produced before etc. ?
Sometimes it
The list of what has never been produced would take a lifetime to list.
Nonetheless, I'll toss out the one item that I would actually buy: the PRR Yellow Kid passenger train. Only done as a set of kits in HO from Westerfield. It might just check a lot of boxes off as viable in O scale with the engine already out there and available. 
