Yet another idea:
An extensive harbor/seaport railroad with curves so sharp docks and many industries can only be served by 0-4-0T's (like little joe). Somewhat larger locos to feed them from smaller yards, and of course mainline locos feed the intermediate locos. Lots of trackage in streets, docks with tracks, ships of various types, and industries galore (and everyone loves little joe - I had one many many-moons ago, lol).
Ben
Ben, I did one similar, with a bit of your help several years ago. The Jay Street Connecting RR. Remember the E.W. Bliss buildings, Arbuckle Coffee Headquartes, Arbuckle warehouse and more. There are still some tracks in the streets of Brooklyn from this railroad. Two 0-4-0 locos for power. Only connection to other railroads was via car floats. There were several buildings with tracks inside the building. The Arbuckle Coffee Company had a 9 track yard on the ground floor of the building. And finding brick buildings was not a problem!
I'm currently working on the Portland Terminal Railroad. Serves a business park that covers about 1 sq mile. There are over 80 industry spots for cars. Connection to outside world is the BNSF yard.
Check out http://members.trainweb.com/bedt/IndustrialLocos.html for some very interesting terminal operations.
David