This is my 120' long test load for flatcars. It only took a few tries to get it to work. The piece is 1" x 1/4" basswood. I ran it a couple of times to see where I wanted it to sit, then I drilled holes in the flatcar decks and inserted a .06 styrene rod with about 3/8" sticking out. I then drilled the 2 holes in the piece of wood and gave it a test run. It crashed on the first 36" curve it came to so I added washers between the wood and the cars on the pins to raise it up so it cleared the deck of the middle car. I ran it again and it still crashed on the curve. I made the hole in the wood a little bigger and it made it through the first curve and crashed on the 2nd 30" tighter curve. I noticed at this point that the couplers on the cars were binding causing the cars to come off the track and tip over. I also noticed that there was no slack in the couplers. So I measured the amount of slack between a set of couplers. So to fix this I put the wood in my lathe and milled a slot extending from the holes to the middle of the car plus an extra .01" long. This allows the cars to have all the slack between the couplers. You would just make the slot longer if you had another idler car.
This shows the milled slot in the load
I ran it again and it went all the way around on the main line with no problems. Next trial was in the yard and it ran with no problems on the ladders or any of the other tracks. The tightest curve I have is 24" and it also ran through that. The wood never tips over and rides very well. With the 1/4" thick wood it just come to the edge of center flat car which is only 9' wide. In order to have the same over hang as my passenger cars I can maybe go to to a load that is 3/4" wide. The next step is to make a real load and the supports to support the load with the slots milled in them. The hole in the wood and the slots will only need to be .080 for it to work. The pins only extend through the plastic deck and can be removed so you are only left with a .06" hole. This can be easily disguised if you are going to use the cars for other loads.
This picture shows the pins in the flat car deck and washers.