Automatic Block System Train Control
Automatic block systems began use in Michigan around 1900. One of the first installations was on the Michigan Central from Detroit to Ypsilanti.
ABS signals are typically installed at two mile intervals and monitor track occupancy. If a train is within the "block" between signals, a stop signal will be indicated at each end of the block so that trains won't collide. These systems get even more complicated so that advanced signals slow down trains that are approaching occupied blocks.
Block signals were originally "banjo" style signals on poles, followed by lower quadrant semaphores (later with lights), then upper quadrant semaphores (with lights), then searchlight target signals, and now LED signals. Some ABS signals had as many as 3-4 different lights. A combination of colors and position indicated various instructions to the Engineer and Conductor.
The Pensylvania Railroad, which they self-proclaimed as the Standard railway of the world, adopted a type of signal called a position-light signal. These were usually one color, but a signal would have 3 small lights in a vertical, diagonal or horizontal pattern. This was good considering that 10% of the population has some form of color blindness, but these signals were expensive to maintain and most have been replaced beginning in the Penn Central era. PRR signals were in place on their line from Carleton to Lincoln Yard and the GR&I lines in Michigan, and some variants were used by the roads which were under control by the PRR from time to time like the DT&I.
ABS Lines in Michigan
The following railroad lines in Michigan used automatic block signals:
Ann Arbor Railroad
Hallet Tower (Toledo) to Ferry Yard (Ann Arbor) - used for UM football specials
Detroit & Toledo Shore Line
Main Line, Lang (Toledo) to Dearoad (River Rouge)
Detroit Toledo & Ironton
Schaefer Tower (Dearborn) to D&I Jct.
Main Line, Flat Rock to Diann (Dundee) - Diann is the Ann Arbor RR connection to Toledo
Grand Trunk Western
In 1974 on the Flint and South Bend Subs from Tappan to West Tappen (2 tracks), East Flint to West Flint (2 tracks), from Durand west to McAllister Road (near Battle Creek), from Emmett Street (Battle Creek) to High Street (South Bend)(2 tracks), Olivers (South Bend to Elsdon (near Chicago)(1 or 2 tracks). On the Holly Sub from Clay Avenue (Detroit) to M.A.L. Jct. (Pontiac)(2 tracks). On the Mount Clemens Sub from the Double Track Switch (near 8 Mile Road) to Milwaukee Junction (2 tracks).
Michigan Central/New York Central
Main Line from Windsor, via Detroit, to Porter, Indiana (and Chicago) - before CTC
Jackson to Rives Jct.
West Detroit to Alexis (Toledo)
Bay City Jct. to North Yard
Oxford and Rochester (to protect gravel pit switching in the area)
Paines north to Bay City (via Saginaw)
Rougemere Telegraph (Dearborn) to Porter, IN via Plymouth, Lansing and Grand Rapids - before CTC
Alexis (Toledo) to Midland via Plymouth - before CTC