Rockershovel,
I've only seen two sizes from Williams and its successor Williams-by-Bachman. These would be 15 inches (or 60 scale feet) and 18 inches (72 scale feet).
I'm interested in these cars as well. Here's a partial compilation (and NYC only) of what I've found:
![Williams Madison NYC_2a0 Williams Madison NYC_2a0](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/157406524505895574/filePointer/157406524513738532/fodoid/157406524513738527/imageType/MEDIUM/inlineImage/true/Williams%2520Madison%2520NYC_2a0.jpg)
Be aware that Williams in notorious for changing its numbering and packaging schemes frequently throughout it's long history. Unlike Lionel, which has used 6-xxxxx numbering for most of the last 50 years, and MTH, which uses 20-xxxxx-y and 30-xxxxx-y, Williams' product number formats are ridiculously unpredictable, hence can be hard to search for on the web.
Also, some sets have no set number because they were sold with neither a product number on the box, nor a label on the box, as you'll note when you look at the list.
I believe that most sets under the Williams-by-Bachman name are still in production and can be found in stock on a number of retailing websites.
Good luck with your search.
Mike