I have had great success using a 12VDC / 5Amp power supply to run everything from prewar Marx and American Flyer to a top-heavy post war Lionel 1656. However, since you own vintage Lionel AC transformers, consider purchasing a Lionel AC to DC Converter such as the 6-5900 (approximately $10. and available on that auction web site). These are simply wired between your transformer and your track.
Early post war motors are universal. They’ll run on AC or DC. The difference is in performance and control. With DC, a young engineer can make his or her train run at a realistic crawl or smoothly increase the speed safely without the need for supernatural fine motor coordination.
As long as the train being run does not have any devices designed specifically for AC operation (e.g. a whistle or bell tender) and the layout is a simple loop, there are no downsides to using DC power.
