The 3811 is an operating version of the 2811 Flatcar. The car came with logs and a 160 bin. The log car was designed to be used with the 164 operating log loader.
The 3811 comes with either a flat frame or a dished frame and was cataloged by Lionel from 1939 to 1942. Both versions are quite common, but the dish framed log car runs better.