18" curves are tight. While many locomotives will supposedly go around them, it isn't easy. It is especially bad with full length passenger cars.
With the space you have, I'd scrap the modern theme entirely and switch to an early 20th century theme. I'd use 15" curves and have a roster of Athearn/MDC 34' passenger cars and 36' freight cars. I'd use various small steam (such as Spectrum's 4-6-0) and if you want diesels, MDC's old boxcab. With tight curves, normal length cars and trains are problematic. The real advantage here isn't in steam vs. diesels, but rather in the super short cars. Short trains with SW-7s and 40' boxcars would be fine...but it'll look really odd.
Importantly, you're going to have a nearly impossible task of building a decent layout for post-steam trains...especially with islands...since it is tough to turn trains with your space limitations.
I would recommend something like this layout.
The mainline is for continuous running. Along the side of the room there are always two tracks with a loop at the top and a loop on the island at the bottom. The weave back and forth a little. On the top island is where the mainline runs out with a scenic divider in between to make it appear to be by itself. Where this connects up with the rest of the layout is a grand trestle over the lower track(s). The middle island is a branch line, including a switch back into the mountain. A small yards would be on the bottom peninsula. There would be small towns at each loop, on the top island, and along the branch line. The branch could be to either serve a town or a special industry (mining, logging, etc).
A trip over the main would be like this:
Train #1 pulls out of town 1 moving clockwise around the loop. It heads up grade and crosses the other track coming out of the town (either with one of your diamonds or a bridge). It heads out on the peninsula to town 2. While there, it weights in the hole for train 2. It then continues on up grade and crosses the railroads most fantastic trestle over some other tracks and enters a tunnel. It then begins its downhill journey past an industry and into town 3. The engine is serviced in the yard and receives an additional car. It then continues out of town to the junction. It then begins an uphill climb to town 1. Train 3 leaves town 3 after train 1. It turns at the junction and heads out on the branch. It negotiates the switchback and visits town 4/or the industry. It then returns to town 3, but has to wait for train 2 before it can enter the main.
The key is the tighter radius allows for both continuous running and a substantially longer mainline. Small power, small cars, and short trains make the mainline seem substantially longer than it is.
As for the tighter curves, you can either purchase them as sectional track or use flex track.
I get the impression that you haven't really built a layout before. I'd suggest just building a small, stand alone portion...such as a branch line or a learning layout. There are a number of lessons you'll learn along the way to make it far, far easier. I say this because your description was of a layout to watch trains run...and you listed a few specific sectional pieces as your track work (such as 18"R curves). That is fine...and I encourage you to think about a long term project such as this, but it is also important to make sure that you know that it isn't a fast process. Further, there are many mistakes that everyone of us makes and then wishes we'd learned them on small projects rather than on a full layout.
You may find that if you want continuous running with Dash 8s and 4-8-4s, N scale might make a better choice.
Note: 18" curves require more than 36" of width. 22"R became standard because it would fit on a 4'x8' sheet of plywood.