I live in the Detroit metro area so that currently has my attention. It is really an interesting rail network because of it's industrial history, though only a shadow of what it once was.
The Holly Sub was GTW right-of-way until the turbulent Conrail takeover/bankruptcy thing that led to the GTW lines and equipment being parted out to several of the remaining class I carriers. A lot of it was retained by CN.
Typically Detroit by definition is a stub line, with the through trains running across Ohio and Indiana, but CN is in a unique position of incorporating it into their major East/West main flow. Their line Westward from Toronto splits, with part going to Windsor, ON (bordering Detroit) and the other part going to Sarnia, ON (bordering Port Huron). The Windsor branch crosses the Detroit river by tunnel into the CN Detroit yard, which is the beginning of the Holly Sub.
The Holly Sub takes CN trains up to a little town called Durand, MI, where it rejoins the Tronto/Sarnia/Chicago main to Chicago.
I live within sight of the track, and it is funny seeing CN freight trains traveling North, bound to Chicago, and the Amtrak Wolverine traveling South, bound to Chicago. Obviously UP, NS, and CSX enjoy the more direct route, and Amtrak gets to take advantage of all of them.
My goals for the line are as follows:
Model track/terrain/city from the CN Detroit yard to Durand (Approx. 80 miles)
Make the engineer feel like he has been to Michigan
Create scenarios for:
Amtrak Wolverine
Through Freight
Local industrial switching including especially
The CN Detroit (Ferndale, really) intermodal facility
So, you know, I might need to model some gun shots and screeching tires.

Right now I have most of the track laid from the Milwaukee Junction to Pontiac, which is actually about 1/3 of the sub. Being local, I can scout around and take pictures and such. I'm also a ham radio operator so its fun to listen in on the Dispatch and CN road frequencies.
I could probably do the entire thing with stock and share-alike models, but I'd really like to make some models just for the area. I used to do some modding for MS Flight Simulator so I think I have a shot.