Baldwin made a number of switchers with similar dimensions and body styles.

The first body style, used in VO models, had a slightly pointed nose with a round radiator opening.

The second and third body style, almost indistinguishable and used interchangeably, had a flat nose and rectangular radiator opening.

The first units (56 locomotives) were powered by an 8-cylinder normally aspirated prime mover, but from 1948 a change was made to a 6-cylinder turbocharged engine.

SMS Lines owns 9 S12's and operates #301 at the Penn Warner industrial park in Morrisville, Pennsylvania, as well as #304 on the Woodstown Central in Salem County, New Jersey with 7 others in storage or awaiting restoration.