Major freight traffic flows in the 1940s and 1950s were along the Trunk Routes, often named for the passenger trains that ran on them at the time, but sometimes named for later different major freight traffic.
Most such traffic simply ran in general merchandise trains, comprising whatever cars were available in the originating yard for the destination(s) served by a particular train, with the cars sorted by destination, either at the originating yard, or at sorting yards en route. Major exceptions to this were unit trains (although they weren't called that, then) for minerals, especially coal, but also ore, livestock (cattle, sheep, hogs, etc.), and agricultural products, including 'perishable blocks' of refrigerator (reefer) cars carrying produce, potatoes, etc., from harvest to market.
Traffic intended for destinations not served by the originating (or intermediate) railroad was interhcanged to anotehr railroad for movement onward towards it destination, or for delivery to that destination. (This still occurs in the 1990s and 2000s, but the number of railroads carrying trunk line freight is much smaller, and thus the number of inter-line interchanges also much smaller.) Major locations for such interchange are among the Regions where many railroads came toigether.
Early mergers had little impact on freight traffic flows, except to increase the number of flows that took place completely within the boundaries of a single railroad, and perhaps divert some once-connecting flows from now-competing neighboring railroads. This was true as late as the 1964 merger of the Norfolk & Western and Nickel Plate (and the railroad's simultaneous lease of the Wabash). On the other hand, it began not to be true with the merger of the Erie and the Delaware, Lackawanna & Western, in 1960, which resulted in the demise of one of the parallel lines between Binghamton and Buffalo, NY. It ceased to be true, with great impact, with the creation of Conrail in 1976, one of whose purposes was to effect the controlled discontinuance of hundreds of miles of redundant and duplicate trackage in its operating area.
The dismemberment of the Rock Island and the Milwaukee Road, in the early 1980s, saw the divestiture of many more miles of duplicate routes, mostly to the new breed of regional railroads, but the mega-mergers since then: the creation of CSX and Norfolk Southern; Union Pacific absorbing Missouri Pacific and Western Pacific; Southern Pacific and Rio Grande; Burlington Northern and Santa Fe; Chicago & Northwestern and Southern Pacific into Union Pacific; Illinois Central, and Wisconsin Central into Canadian National; and the split of Conrail into pieces absorbed into CSX and Norfolk Southern, have largely reverted to the former impact, that of increasing the number of traffic flows wholly within a single railroad, and the increased efficiency of many flows that formerly required the cooperation of more than one railroad for their completion, with some concomitant losses of traffic to former connecting railroads.
Some of the impacts of the mergers of the second half of the 20th-century are described here.
The number of routes used by major freight traffic flows in the 1990s, and even more in the 2000s, after the major mergers of the late 1990s, has been reduced quite considerably, and traffic flows now travel routes differentiated in large part by the major traffic carried. The separate traffic flows that have been identified, and are covered here to varying degrees, are:
Steel
Oil and other tanks
Livestock
Agricultural Products (excluding grain and livestock)
Local Peddlers
An overall summary of the changes in traffic flows between Chicago/St. Louis and the Northeast from the New York Central and Pennsylvania Railroad era, through the Conrail era, and into the devolved Norfolk Southern and CSX era, is here.
An overall summary of the traffic flows between Chicago and the various destinations and origins in the Southeast is here.