The transition era lasted for ten or fifteen years around the second world war, and consisted of the replacement of steam power with diesel power. Some rail lines did this very quickly, some did it gradually as diesel power matured.

The picture below shows four Sante Fe F units, one of which is an "A" unit with the cab and distinctive War Bonnet paint scheme, and the other three being the "B" unit, which lacked a cab. The locomotives are pulling a passenger train.