I'll be 72 in March. I learn new things almost every day and hope that never stops. While I'm getting pretty good at using SCARM, I have a hard time coming up with ideas for cities and industries. I grew up 30 miles south of Green Bay and my only experience with railroads was the cabooses parked at a siding 300 or so yards form one of my childhood homes. I road the trains a few times from a small town called Brillion to a coastal town called Manitowoc on Lake Michigan. I never got to the point where I had enough space for a layout and still don't. My current project is building a showcase for my wife's Bedford Falls collection she got a long time ago from Target.
I got back into O Scale in 2012 and have had a number of setbacks getting something built. I've had rather large Christmas layouts (8x14), but nothing permanent. I've designed 5 different layouts for 2 bedrooms and part of the garage. I've even rearranged the garage for ta layout, but travel and a few medical issues kept delaying the build. Now I'm have issues with my sciatica that have affected my walking, so I'm having 2nd thoughts on the garage altogether. Between the heat and dust here in Phoenix, I think I'm going to concentrate on getting Bedford Falls up and running in the craft room.
It'll consist of oversize 4x8 tables butted to form an "L" in one corner of the 10x21 room. Since I really just like to watch trains run, it will have 3 simple ovals of various designs on 3 levels with the Bedford Falls buildings scattered around. All the excess buildings will be put on display shelves. It will be on wheels, so I can move it if I need to get behind it, but with no switches, I don't think that will be needed very often. I have a Christmas 4-6-0 steamer w/passenger cars I want to run during the holidays and a Veteran's SD70ACe diesel w/freight cars. I plan to add another diesel and maybe an old-style General 4-4-0 steamer w/passenger cars to run year round. It'll be DCS/TIU controlled and that's about it.
As for industries, there are the tried & true logging, coal mining, mineral mining and oil. You mentioned a subway rising up for a bit and that will kind of require a rather large cityscape, perhaps along the left wall. And since you mentioned lower level subway, you need to plan for elevation grades and those need to be 3% (3" in 12'), preferably closer to 2% (3" in 8'). The MTH graduated trestle set is made to rise 6.5" for a 4.7% grade (6.5" in 11.5').You need around 6." of clearance to account for track height and roadbed thickness. Of course, the subway will can be on it's own track below grade and you can hide the rise behind landscaping to achieve the look you want. But, it can take a lot of track below grade. Here's an example. Note that the Blue tracks in the Red circle do not clear the Purple tracks on top. That means the grade has to be started after the Purple tracks and then the grade will be steeper than most of is like, from 2.9% to 5.4%. Now, modern equipment can probably handle that, especially if the subway is limited to something like 3 cars. I'll let you look at the 3D view in SCARM.