Things to consider,
First off, batteries aren't an awful route, especially outside. But I can't stand charging chores. If I forget, I'm screwed for an hour. Even in RC I started with batteries, and went to fuel for that reason. Better today, sure. But batteries don't last forever and aren't cheap by my view, it's an additional cost long term I don't want. Not to mention you can dump as much into the radios as a fully bloomed Legacy command (I didn't even hug a tree or step into a true carbon footprint 
) Know yourself before you leap into batter op. deep end. Maybe dip a toe if you aren't sure.
The 24" inch drop is going to need a ton of length to have a reasonable downgrade run. And longer if you want to climb it again... and longer yet with all those cars. (I didn't follow on the exact return to the house, if any)
7" rise/drop at 15'+ long, is about a 4.5-5 grade. Ok for short trains with a decent 1 motor loco (maybe 12 freights). That's about 50' for a 4 to 5% and You want to be at 2% or less to climb with a huge load....100' ? (not sure offhand, and not up for good math at the moment anyhow)
With steam it also has to drop and stàrt the grade more gradually as the long wheelbase will teeter on an apex, and cowcatchers bottom out on center rails at the bottom (both directions on both issues too) Longer wheelbase 6 wheel trucks suffer that too but less. (0-4-0 steam are best, diesel 4 wh/truck next.)
UV sunlight on plastic ties. It needs protection, some have it some don't. They will fade, and crack in 3-6 years otherwise.
Wood isn't better. Even cleared it doesn't usually do great outside..ties are twigs.
Powered track? You want stainless or good Nickle silver. You'll have to research both, I forget the exact ins and outs of rust, vs corrosions vs resistance in the two.
The resistance is definitely higher than 'normal' with those alloys. You'd be smart to run nice fat bus wire in conduit on or underground and using a lot of drops and long 36-40" sections to reduce joint numbers to a minimum.(joints = resistance points)
You didn't mention the loco(s) you might use. This has bearing on how many cars you might pull. Heavy pulls more, rubber traction tires too. (Magnetraction needs plain steel)
Car shells; plastic or metal? You can always pull more plastic ones. (a UV clear might be a good idea. Even paint fades in sunlight)
Lit cars? That means rollers. Shoes have better contact, but the drag would be huge at 12+cars.
Use led lights, you do want to be power conservative here with such distance. (might even want to use a capàcitor with them to stop them from going out immediately if you throttle down. They can be made to light for a few minutes after power is shut off. They will fade to dark too.)
