I have a ton of the RMT cars, when they were $10, I bought some. After I got them, I realized what a great deal they were and scale sized as well. Then I bought a WHOLE lot more. Like David, I still have a few unopened cases of them, I couldn't see how I could go wrong.
I have some K-Line and Lionel ones, and even a couple of the MTH ones. All are very close in size and quality, but you'll likely never see $10 NIB ore cars again, especially of decent quality like the RMT cars.