The first generation truck was developed in 1975 and appeared in the 'Repli-Kits' of the Madison cars when they were released, in late 1975 or 1976. The second generation truck appeared in 1978 (or a little earlier). It had less parts and was easier to assemble.
The problem with Williams production is the lack of records that are available, if they exist at all. The picture of the Amtrak FM and Amfleet car that I posted earlier is like the car you have. That was the 1981 catalog. The difference between your car and the car in the picture is the truck. The first trucks on the Amfleet cars were Lionel.
The Metro Liner was going to use the trucks off the Lionel Coke diesel. Apparently the Coke set did not sell as well as expected resulting in large overstocks. Dealers were willing to break up the set and sell the Diesels(or parts) to Williams on the cheap. With just the one powered truck on the Metro Liner it did not operate properly therefore they used two powered trucks. This left two unpowered trucks for each Metro Liner. Williams being the ever inventive guy put the unpowered Lionel trucks on the Amfleet cars. However, because of the construction of this truck there was a lot of friction. When they ran out of the Lionel trucks they used their own truck. Your car was probably made after they ran out of Lionel trucks. 1981 forward. Not sure how many sets got made. I think I read that there were under 300 Metro Liner sets made.
I have posted picture of first and second generation Williams truck. Ignore the fact that the guy assembled the first one backwards putting the pickup outside the truck. The Amfleet car picture is over exposed so you can see the Lionel diesel truck.
I think this is all I think I know or it may be more than I know.
One more thing. If you look carefully at the top of the Amfleet car you can see the little plastic plug. They plug the holes that were used to mount the pantograph. The later cars, the ones that were not left over Metro Liner cars because of having to put two motors in them, did not have these holes or plugs.