here is what i have come up with so far. its slightly wider than 3 feet, but only by a maybe 3 inches, barely.
its got an oil refinery ( dark blue tracks) next to a canal, where oil barges are brought in by tugboat. the prototype does this, atleast here in the northeast, because the big tankers can't get into the hudson and other areas around new york, so they unload onto smaller tankers ( to big to model on even medium layouts) or in small barges. you can have it look like a barge is moored nexed the oil refinery. you can use Walthers oil tanks, and a single Tank car laoding platform. maybe even the refinery kit if you can get your hands on one.
across the river you have a residential area to fill in the space. you also get to industrial spurs for whatever building you can fit in there. i suggest the green tracks get a manufacturer (which also has like a dock next to it if you want) that recieves either boxcars, or plastic pellet hoppers, for makeing things. the yellow tracks can be whatever else you want to model. if you make the green tracks recieve box cars, a good industry for they yellow tracks is the Plastic Pellet transfer. i think it was retired, but i still see them around and at train shows, so it might be a good thing to put there.
ther red tracks are supposed to be an interchange/ hidden stageing. they are hidden by residential houses. i suggest getting the book Building city scenery on how to hide tracks with houses and buildings. back at the bottom there is a blue run around track so that you can park your freight cars bound or from for the interchange on the mainline and go around them.
next we have a yard. some of the tracks are coorked, but that is because the track planning software is super exact, and doesn't allow fudgeing. fudgeing is one you tilt or bend a track ever so slightly at the joints between another track, so it lines up. fudgeing usualy isn't harmful, nor will it cause derailments. you have to fudge the track in many cases. in any event, the purple track is where you can park an extra locomotive. you could put a single stall engine house there. there are 3 tracks in the yard for sorting, and an extra lead track so a trai can back into the engine house. the yard is enough to hold 7 50' cars, and a little more 40' cars.
you also get 2 RR bridges, which is nice. all track is made of atlas snap track except for the Custom Line Wye in the yard. you'll need to get a switch machine for it. just about everythign has got its part number next to it.
curves are 18' radius curves.
this layout is best used with a B23-7, GP38-2 or similar, as well as smaller switchers. you might be able to pull off an SD40-2 if you really want to, but I wouldn't get any bigger than that. now the picture turns up as a box with a red x in the box you type in, so i don't know if it will show. if it doesn't i'll email it to someone who can make it show up.