No, MTH did not separately sell the mid-car units (C, D, and E). You have to buy the five-unit sets mentioned above. They’re nice sets, but have limitations. The hitches don’t fold; indeed on the A, B, and E units they’re cast into the body in the retracted position. So you’re limited to carrying only two trailers on units C and E as shown in the pictures. I modified one of my sets to carry a third trailer, but it involves cutting out the cast-in hitch and mounting a replacement.
K-Line also made five car sets with retractable hitches. These can carry any combination of containers or trailers. The mounting pins probably limit you to K-Line containers, but they can carry pretty much any O scale trailer.
Here are some videos of both an MTH set and a K-Line set in action. All equipment is converted to scale couplers for true thee rail scale.
RM