And I know nothing about how long Madison cars are. All I can tell you is that Lionel called the early Bakelite cars "Madison" and the name became generic for "heavyweight" style cars. If you are not doing 0-27, best to skip the "Madison" label and use the names given to the prototype - e.g., Pullman, Harriman, etc.
18" cars are 72 scale feet long - plenty long enough for commuter lines. 20" is about as long as you want to go for heavyweights. SP Daylight "lightweight" cars started at 77', which is 19 1/4" in O scale. Does any of that help?