Concur with Mallard. Literally everything made by Lionel between World War II and 1988 will run on O-36. That includes the Scale Hudsons from that period, too. Most will also run on O-27. From 1988 to 2000, almost everything will run on O-36.
For the most part, MPC re-used postwar tooling, so the MPC Berks and (non-scale) Hudsons are based on postwar models: 736, 646, or 2065. The MPC steam passenger sets used the 13-inch "Baby Madison" cars, which were all-new tooling by MPC. The 14.5-inch postwar-style Madison cars were reintroduced by LTI in the 1990s. Those also look good with MPC steam.
Like postwar Lionel, MPCs rolling stock was a mixture of scale (or nearly scale) and decidedly undersized, yet most of it looks pretty good together. Certainly, anything originally sold or marketed as a set will look "right".
I would recommend getting a Greenberg's Guide covering this period. It will orient you to what is available, and with some study you will develop an eye for the postwar / MPC "look," which will help a lot when it comes to knowing whether the train in front of you will look correct with what you already have at home. The best (in terms of number of photos and pages in color) is 1970-1991, Volume 1, but there is a 1970-1987 edition which is adequate, and also probably a little cheaper.
