The queue on the 60 foot boxcar may allow 140 containers, but that would be 140 containers that weigh no more in aggregate than 1/2 ton each (if the car is a 70 ton car). if the containers of general goods weigh nearly three times that many, the car would be loaded with one third the number of containers. To determine the number of containers that can be loaded, divide the load limit (LD LMT) number on the side of the boxcar, by the weight (mass) of the containers to determine the maximum legal load.
It is possible, too, for the dimensions of the car to allow the loading of far more containers than the weight of the containers would permit. For example, I once visited a shipper of aluminum ingots who was using boxcars. The weight of each ingots was 15 tons, and the size was such they were loaded, three ingots in each end, over the trucks, and no ingots in the center by the doors. The ingots only required about 1/6 the volume of the box car. Loading the car by volume would have destroyed. the car.
I would note that the calculations John posted are correct, but probably do not apply to the cars in game; while there are likely today boxcars which are constructed to the 110 ton weight standard, that weight standard was not widely used in the era that cars were being purchased by SP (though there may have been a few). More likely, the 60 foot SP cars (if you look at them closely) have a 70 or 100 ton weight limit, and should not be loaded with more than that weight. In the real world, if the railroad found that the car was overloaded, if it had not departed the initial terminal, it would have been returned to the shipper to have the load adjusted. If found away from the original terminal, the portion of the load causing the overweight would have been removed from one car, loaded into a second, before forwarding. Bulk trains today--e.g. coal, sand, plastics, &c.--are typically loaded by weight very carefully by the shipper, as the revenue the shipper receives from the consignee is most likely weight based.
ns
