The Soo Line Railroad is one of the primary United States railroad subsidiaries for the CPKC Railway, one of six U.S. Class I railroads, controlled through the Soo Line Corporation. Although it is named for the Minneapolis, St. Paul and Sault Ste. Marie Railroad (MStP&SSM), which was commonly known as the Soo Line after the phonetic spelling of Sault, it was formed in 1961 by the consolidation of that company with two other CPKC subsidiaries: The Duluth, South Shore and Atlantic Railway, and the Wisconsin Central Railway. It is also the successor to other Class I railroads, including the Minneapolis, Northfield and Southern Railway and the Chicago, Milwaukee, St. Paul and Pacific Railroad. On the other hand, a large amount of mileage was spun off in 1987 to Wisconsin Central Ltd., now part of the Canadian National Railway. The Soo Line Railroad and the Delaware and Hudson Railway, CPKC's other major subsidiary, presently do business as the Canadian Pacific Railway (CP). Most equipment has been repainted into the CP scheme, but the U.S. Surface Transportation Board groups all of the company's U.S. subsidiaries under the Soo Line name for reporting purposes. The Minneapolis headquarters are in the Canadian Pacific Plaza building, having moved from the nearby Soo Line Building.

A reporting mark is a code used to identify owners or lessees of rolling stock and other equipment used on certain rail transport networks. The code typically reflects the name or identifying number of the owner, lessee, or operator of the equipment.