Winter Service, photo courtesy Mackinac Bridge Authority

The Michigan State Ferry Album says:

Winter service began in 1931 when the Highway Department arranged with the Mackinaw Transportation Company to carry cars across the Straits on a railroad icebreaker during the cold months. This arrangement turned out to be poor business for the State, so in 1936 the Highway Department leased the railroad icebreaker “Sainte Marie” for winter operations on a regular schedule.Winter service began in 1931 when the Highway Department arranged with the Mackinaw Transportation Company to carry cars across the Straits on a railroad icebreaker during the cold months. This arrangement turned out to be poor business for the State, so in 1936 the Highway Department leased the railroad icebreaker “Sainte Marie” for winter operations on a regular schedule.

In case you’re wondering, the ice on the Straits of Mackinaw hasn’t changed a whole lot in 80 years, as this photo from February of 2008 titled Triangles by Dominique shows. See it bigger in her Snow/Ice slideshow or check out the whole set.