signalblogSince its birth, the railroad has employed a variety of signaling methods to communicate with locomotive crews as to what course of action they must take, mile by mile, in order to safeguard themselves from possible collision.

In the early years, when railroading was in its infancy, one train might be the only one running on a specific track for a day or more. During this time, rail signaling reflected the form being used in England and parts of Western Europe. These countries were using steam power, a copper ball and a flagpole. The copper ball was raised when a train was fueled up, passengers and freight were loaded, and the track was properly switched. This “highball” was the “ready to go” signal.

As rail traffic increased, however, multiple trains began to use the same track. As a result, there became a sudden need for more stringent traffic control, and the concept of a “block” was developed.

The blocking system broke a line of track into smaller segments able to be controlled with signals. This meant, that at a certain interval along a given track, these early chain and ball signals would be placed to secure a section of track along with an operator to maintain the signal. While one train cleared a section of track, another train waited at the signal for it to clear. Then when everything was cleared, the ball signal was raised and the train proceeded.

The dawning of reliable electricity led to the invention of a coded track circuit which used common principles of conductivity. A box of circuits and electromagnets called a “relay” was placed at each end of a section of track. Each rail was then electrified by a supplied current. At the ends of each section of track, or “block”, a strip of insulation was placed between the rails so that the next block could have its own circuit and not interfere with the circuits of surrounding blocks.

When a train passed into an electrified block, the circuit from one rail would travel over the steel axles of the train to the other rail and create a connection. The relays would then detect this loss of electricity and a series of electromagnets would become demagnetized. This created a new circuit that then directed power to the railroad signal which rotated a pivot and illuminated a lens from green to the red. Thus creating electrified block territories that are still used today.

Some track territory featured more than one track, with some tracks going in all different directions, such as yards, crossings with other railroads, or high traffic regions. This was where manned signal operators remained necessary. Manned interlocking towers were used on the railroad to control these points. Each tower was given two letters to identify itself on the telegraph wire. The letters usually involved some relation to the name of the town but were ordered so they weren't confused with other letter codes used on the telegraph. For example, the tower at Alexandria positioned to control movement to Fredericksburg was identified as “AF”, or Alexandria-Fredericksburg (currently the dividing point between Manassas and Fredericksburg lines). Each tower operator was responsible for switching the appropriate tracks by hand and telegraphing the dispatcher when a certain train had passed.

In the mid 20th-century, Centralized Traffic Control system (or CTC) was developed. This was a large console with a series of lines depicting tracks, switches and other miscellaneous track structures. At each track switch depicted on the console, there was a small light bulb and a small lever. When the light bulb was lit, that meant a train was occupying that “block” and if the train's destination required transfer to another track, the operator, miles away could simply turn the lever or push a button and instantly a signal created an impulse in a relay box that then in turn operated a motor and switched the track. With this amazing new technology, manned signal towers were no longer needed, and the railroad companies began to demolish some of these towers and installed traffic control consoles in centralized locations.

With some technological advances in the signal systems themselves, this is largely how our trains are dispatched today.