For a while in the 1970s and 1980s, Lionel used DC power in its least expensive O27 electric train sets. They stopped this practice in the mid 1980s, but there are still plenty of those sets kicking around in basements or attics and on the secondary market. They tend to be very inexpensive, especially compared to new sets on the market today.
Here’s how to figure out what you have, and track down a suitable replacement. AC and DC power supplies are not interchangeable, and you can seriously damage your train if you use the wrong kind. Trying to run a DC train on AC results in a train that won’t move and makes a ton of noise.