The train dispatcher is the most common heir to the towerman’s duties, but not always. The type of control used depends on the nature and density of the rail traffic handled at the location.
Ways to preventing trains from colliding when railroad lines cross or meet
The most simple solution is to have the trains stop while a crewman operates the necessary track switches from the ground. Obviously this procedure works only at locations that now have just a few, unexpedited trains because of reduced traffic.
If the former interlocking tower protected the crossing of two railroads, they may erect a gate which is swung by a crewman across one line or the other. Under such an arrangement, trains of either line approach the crossing prepared to stop if the gate is across their track. The timetable specifies which road the gate is to be lined across when not in use. If one or both lines are protected by automatic block signals, the gate is electrically locked and the crewman must activate a timer before the lock is released.
Even more simply, some crossings are protected only by four-way stop signs. Again, these solutions work only on lightly traveled lines.
An automatic interlocking may be installed to protect main track crossings where there is little or no connecting through traffic, typically in rural areas. Automatic interlockings operate on a first-come, first-served principle. When a train is sensed to be approaching (by the shunting of the track circuit), signals are cleared for its passage. Any other approaching train will encounter a red signal until the first train is clear of the crossing.
At locations with high traffic volume, the tower may be gone but the interlocking system remains, remotely controlled from another location. That location may be an open tower several miles away (typical in terminal areas) or the dispatcher’s office perhaps half a continent away.
The General Code of Operating Rules defines an interlocking as “signal appliances that are interconnected so that each of their movements follows the other in proper sequence.” When they were mechanical, these appliances necessarily had to be operated by someone nearby, i.e., the interlocking towerman or operator. Now electronic, they may be operated from anywhere.
An interlocking may be linked to a railroad’s automatic-block-signal (ABS) or centralized traffic control (CTC) system. ABS provides automatic control of signals; CTC includes automatic control of signals plus remote control of signals and track switches.
There is no fundamental difference between an interlocking and centralized traffic control. Typically the interlocking may have a more complex track arrangement, but the rules governing both are the same.
Automatic block signals
ABS has its origins in 1872, when the electric track circuit was invented. By dividing a track into separate sections (“blocks”), which are insulated from each other electrically, and making the blocks part of electric circuits which include lineside signals, a system of automatic train protection was achieved.
If a train is in a block, it completes a circuit that automatically sets the signal behind it (and ahead of it, if the line is signaled for movement in both directions) to red, warning any approaching trains to stop.
ABS was a major advance over manual block operation, which required a person (“block operator”) working under the direction of the train dispatcher to grant authority for trains to enter the blocks under his control.
Operators were still required, however, to set track switches at junctions and crossovers; the train crews themselves would generally operate switches at passing sidings.
Double-track ABS is like a two-lane highway with no passing allowed; it works well if all trains run at the same speed, but fast ones can get stuck behind slow ones until the slow ones “pull over” into a siding.
Centralized traffic control
CTC dates from 1927. Sometimes called TCS (for Traffic Control System), CTC puts control of signals and switches in the dispatcher’s hands, eliminating the need for operators and enabling trains to meet each other without stopping for their crews to throw switches.
It’s more expensive to install than ABS, but it has great benefits in terms of capacity and efficiency. Many railroads have replaced double-track ABS systems (each track signaled in one direction only) with a single track equipped with CTC and passing sidings. Single-track CTC has about 70% of the capacity of double-track ABS and offers savings in maintenance, taxes, etc.
Lines with two tracks and CTC are like two-lane highways on which passing is allowed: with the tracks signaled for movement in both directions, the dispatcher can run a fast train around a slow train without stopping either one. What allows trains to take advantage of the two-lane highway are interlocking crossovers that permit movement from one track to another.
Conceptually, CTC is simply a series of interlockings strung out over many miles, operated from one remote location. Under CTC, these “interlockings” are often called Control Points, defined as the locations of absolute signals controlled by a control operator, who is any employee qualified and assigned to operate a CTC or interlocking control machine (dispatchers, operators).
The term has even altered place names: the busy Indiana junction at Milepost 482 on Conrail’s Chicago Line is no longer “Porter,” but “CP 482.”
Control Points, like interlocking machines in towers, are designed so that each of their movements follows the other, preventing the establishment of conflicting routes and other unsafe conditions.
Besides protecting against route conflicts, interlockings must prevent the throwing of switches under trains, and have timing devices which prohibit a route from being lined up until any conflicting moves approaching at maximum speed have had time to stop short of the plant.
Relay systems have been standard for providing control at remote interlockings, but recent installations have used microprocessors, which are less costly and vastly smaller than relay systems.
As you can discern from this, the dividing line between interlocking and CTC is a fuzzy one, with most of the operating concepts and practices overlapping each other. So it is not surprising that most of the functions of the tower operators are now performed by the train dispatcher.
Today, metal boxes (called “bungalows” for their likeness to small houses) containing relay equipment stand at most interlockings.
Zoo Tower in Philadelphia has long been seen as the ultimate in complexity. But the largest interlocking “towers” of all may really be the systemwide dispatching centers established by railroads such as CSX, Union Pacific, and Burlington Northern Santa Fe.