Projecting an image of style and modernity, CN used the five locomotives 6400 to 6404 regularly for publicity purposes. (Brochure C2126)

Canadian National Railways No. 6400 (670001) is a remarkable artifact measuring 29 metres in length and 4.6 metres high. Rather than standard engine black, it is painted in CN olive green to match the colour of CN's steam era passenger cars. The locomotive's distinctive semi-streamlined design was the result of extensive wind tunnel tests undertaken for CN by the National Research Council beginning in 1931. The research sought to improve the efficiency of locomotives and to prevent the recurring problem of smoke obscuring the view from the cab. After testing a variety of wooden models, the NRC researchers arrived upon a design that reduced resistance and eliminated smoke eddies at higher speeds. CN 6400 was one of an order of five locomotives constructed by the Montreal Locomotive Works using this semi-streamlined design.

Canadian National introduced the five engines with considerable fanfare in 1936. Their distinctively modern look made them a natural marketing tool as the company sought to encourage railway travel during the Depression. The engines were put to work in passenger service, primarily in southern Ontario. The year 1939 was a highlight in 6400's career as it was one of several locomotives chosen to pull the royal train carrying King George VI and his wife, Queen Elizabeth, over CN's portion of their cross-Canada tour. Later that same year, the locomotive was featured at the World's Fair in New York City.

Canadian National painted CN 6400 in a distinctive royal blue and gold livery for the 1939 royal tour. Royal insignia were applied to the nose, running board and tender.(CSTM/CN Coll. EH-66-2; EH-66-3)

For all of its distinctive characteristics, the locomotive is a representative example of a CN locomotive. It is one of the 243 "Northern" type locomotives that Canadian National preferred to use for its passenger and freight services. Engines of this type were equally at home hauling twelve-car passenger trains or eighty-car freight trains. The rounded Vanderbilt style tender carrying coal and water for the engine was an efficient design used on most of CN's larger engines from the early 1920s onwards.