Trains and Railroads

In the early days of railroading, switches were set by hand and signal systems consisted of flags during the day and lamps at night. In 1856, an interlocking signal was designed to prevent signalmen from setting signals and switches in conflict with one another. In 1865, Ashbel Welch of the Camden and Amboy RR developed a new type of signal known as the manual block-signal. Trains were spaced apart by a prescribed distance or "block," and new trains could not enter this block until the first train left. The electric telegraph was used to pass the word that track was clear and the train ahead had reached the station.

Switch and train location information was conveyed to engineers by stationary signals such as flags, patterned disks that rotated between full view and edge view, or fixtures with semaphore arms. In 1871, electrical control of block-signals was introduced. In 1890, a compressed air switch with electronic control was installed on the Pennsylvania-Baltimore & Ohio railroad crossing. Fully automated switches soon followed in which the train wheels and axels made a complete conducting circuit, operating relays that ran the signals. The colored electronic lights used today are modern versions of these early signals.

Modern switching yards are largely automated. Car speeds are computer controlled and switching is automatic. Meanwhile, sensors and detectors check for loose wheels, damaged flanges, or other faulty equipment.