I haven't seen this subject in print although someone has surely written about it before. This is based on my own observations.
Postwar Lionel freight cars generally have the last digit of the four-digit car number corresponding to the car type as follows:
- - - 0 crane or work caboose
- - - 1 flat car
- - - 2 gondola
- - - 3 rarely used for freight cars
- - - 4 box cars
- - - 5 tank cars
- - - 6 stock or hopper car
- - - 7 caboose
- - - 8 box cars
- - - 9 work caboose
There are various exceptions but this is the general pattern for freight cars. Some other patterns are:
Freight cars 2411 - 2855 had coil couplers (1945-1948)
There are relatively few car numbers in 4000 and 5000 series
6800 - 6844 are 10" flatcars with various loads (the highest numbers for postwar Lionel)
I wondered if maybe the 4000 series numbers were reserved for car numbers for additional "electronic" sets that were never built.
Additional info and comments welcomed.