Kevin, my test was with pretty thick 6 inch steel brackets. You can get a good idea of them from my first pic on 11/18. I've always advised to test with your hands and see if you can bend them. If not, you might try a hammer to make sure, which I did, haha! Your 12 inch boards should actually be 11 1/2, so a 10 inch bracket that will bear 100 lbs should be fine.
I have no experience with MDF, all my overhead layout is pine. I would advise you to avoid much bowing! That will lead to your trains derailing, probably sooner than later. Simple changes in grade are okay. Beside the obvious danger of trains falling, you don't want to be frequently getting on a step-stool to put trains back on their tracks. Also, derailments have the potential to harm the electronics in your engines. Make sure you install fuses or breakers between your track and your controllers.
You may also want to shim some of your corner tracks to help avoid derailments. I have several turns, and had to do it for the inside line in one of them.
Since my boards are all doweled, they are pretty strong. I run two ABA engines with over 25 cars apiece on the outside line. The inside line has a single Amtrak with 10 passenger cars, including 4 Superliners, and a single Conrail that is pulling 31 cars. That may seem like a lot of weight but, my main concern is weather the CARS can pull each other, rather than the engines or the boards handling the weight. Based on what you've told us, you shouldn't have any weight problems up there.
No, I never remove any cars because of weight concerns. If you grab a board and pull down on it, it will bend but, they NEVER bend because of the trains.
So, you won't be assured until you mount a board. One board is all you'll need to tell you what you want to know about the weight.
As always, good luck and keep us posted.
Jerry
