I should preface this by saying that a complete answer would require about 100,000 words. I am not exaggerating. Here is the short answer, which leaves out so many important aspects it’s probably completely worthless. I have more than 100 books on railroad rates and regulations on my home bookshelves, but none provide a simple answer. (I looked.)
The only thing that is easy to define is the boundary of the Official Territory. Moving counterclockwise, it was bounded by the Atlantic Ocean, the international boundary with Canada, a line generally south of the Santa Fe from and including Chicago through Streator and Joliet, to and including Peoria, to and including St. Louis, the north bank of the Ohio River, and generally the lines of the C&O to North Kenova, Ohio, and the N&W through Roanoke to and including Norfolk, Va.
The Official Territory dates to the practice of pooling and traffic associations, which were formed by railroads in order to maintain rates, provide uniformity and predictiblility to shippers and railroads, and avoid ruinous competition. (First was the Iowa Pool, of 1870.) Following a rate war in 1877, the four Trunk Lines (which by definition are the New York-Chicago principal routes), the Erie, NYC, PRR, and B&O, formed a pool.
These are called trunk lines, by the way, because at first they were almost pure through routes to Chicago, with hardly any branch lines. The feeders that ran essentially perpendicular to the trunk lines, intersecting them at numerous small towns in route, were at the time largely independent. Like the trunk of a tree, the trunk lines gathered and distributed, and ran unbranched in a straight line from one end to the other.
While the ICC Act of 1877 outlawed pooling, it did NOT outlaw collective ratemaking, so the rate bureaus already set up by the railroads continued. The first territory to be acknowledged by the ICC was the Eastern Trunk Line Territory, to which was added the New England and some other terri