The 2811 Flat car was introduced by Lionel in 1938 and cataloged until 1942. It’s the same as the 811 flatcar, but has automatic couplers. It was only made in aluminum with black lettering, and was never included in sets.
The 2811 and 811 while shown in the catalogs in 1940 – 42 probably weren’t made because dished frame versions haven’t been found. The 3811 automatic log car was made in both frame types.