I believe the Lionel boxcars of the 6464 type were modeled after the ubiquitous 1937 AAR boxcar (Association of American Railroads standardized design), which was 40' long.
Detail wise a Lionel 6464 boxcar is sort of a mongrel combination of the Murphy rectangular panel style of roof used in the 1930s and into the 1940s with sort of PS-1 like ends. They were built to be toys and not accurate models. The length is about right for a 40 foot boxcar but the height and width are too low and narrow for a WW I era USRA boxcar let alone a later, larger car like a 1937 AAR boxcar.
How many real freight cars could a real GP7 or GP9 pull? As others have said, that depends on several factors. A single Geep switching in a yard could pull a 60 to 80 car train on the level at slow speed with short periods at high amperage to get the train moving. You would see three or four Geeps assigned to move the same train at higher speed over the road. More Geeps would be needed if mountain grades were encountered.
A modern O gauge Geep model from Atlas, MTH or Lionel with all axles powered and traction tires could probably handle about 40 modern boxcars of any type, including 6464 reproductions, on level track. If you used postwar 6464s the total would be less due to the comparatively poorly designed, higher friction trucks. A post war Geep with magnatraction could pull a lot less and generate a lot more heat doing it.
Another extremely common boxcar was the PS-1 (Pullman Standard), which was built in the 1950's (maybe into the early 60s). Some are still in use today, mostly in Maintenance of Way trains. These are also 40' cars.
Pullman-Standard made their PS-1 boxcars from the late 1940s well into the 1960s. Both 40 and 50 foot versions were available.
Lionel, MTH and Weaver make 1/48 scale 40 foot PS-1 boxcar models. Atlas and MTH make 1/48 scale 50 foot PS-1 models.
The nice photo of the Lehigh Valley car that John posted is of what is commonly referred to as a postwar AAR boxcar. It has a different style roof and ends than the 1937 AAR pattern cars. The Atlas Trainman 40 foot boxcar is the only 1/48 scale model of these extremely common cars.
We now have over 40 different boxcar models available in O scale from 1918 built USRA 40 footers to the Lionel 86' monsters from the 1960s and 70s. And that is not counting traditional sized models like 6464s. It is good time to be in O!