|Tower 1 @ South Station Terminal|
Railroads use a system called Interlocking to control train movements through switches. Interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signaling appliances and tracks are sometimes collectively referred to as an interlocking plant. An interlocking is designed so that it is impossible to give clear signals to trains unless the route to be used is proved to be safe.
When South Station was designed, the railroads were fully aware of the amount of traffic they were expecting to handle once the station was put in full operation. In the first year of operation, the station handled 737 trains daily. By 1913 the station handled 38 million passengers annually which was 16 million more that Grand Central Station in New York. In addition to scheduled trains, deadhead movements, car cleaning, etc. this involved about 2,500 movements through the terminal's interlocking plant daily in order to service this many passengers & trains.
|Track under control of South Station Terminal Interlocking Plant 1899|
To solve the problem of moving upwards of 90 trains per hour, the South Station Terminal needed a large tower to be able handle the enormous traffic load. When the Terminal opened in 1899, there were actually 3 towers that handled all the traffic - Tower 1 (shown here) which controlled the main trackage, and tower 2 which handled the suburban lower loop trackage & interlocking. (Tower 2 was never put into full service as it only handled the underground loop -which was only used only once officially.) Tower 3 was located on the curve just past the Fort Point Channel Bridges. This Tower handled switches at the yard limit for both the NYC and NH approaches.
At the time of the station's construction, the primary method of controlling the movement of the switches was through the use of mechanical levers. This involved a person, located in the tower, to physically pull a large lever mounted to the floor. The lever was attached to system of mechanical linkages to a pipe that ran from the tower to the switch that needed to be thrown. It was all very complex to operate & maintain since all the mechanical linkages tended to jam or would freeze-up during cold weather. It also required a large crew of very strong men to be able to push & pull the levers day-after-day.
|Mechanical Levers for throwing switches|
After much consideration, the engineers selected the Westinghouse electro-pneumatic interlocking system that was recently invented. It was a very logical choice as it required 1/3rd the amount of levers, fewer men to maintain & operate, and most importantly, a smaller tower which could be located in the middle of the station trackage for the best view of the switches & signals.
The system to be used at South Station was manufactured by Union Switch & Signal of Swissville PA. while all switches & frogs were manufactured by Ramapo Iron works. The system was installed under the supervision of George B. Francis South Station's Resident Engineer for the project (information on Design & Construction of South Station written by Mr. Francis himself can be found here: Transactions of the American Society of Civil Engineers Volume XLIII )
The entire system was not operational when the station opened in December 1898 so a temporary shack was built until Tower 1 was completed. On May 7, 1899 Tower 1 & its Model 14 Electro-Pneumatic machine became fully operational.
|Interlocking levers & locking bed|
Tower 2 was built to control switches & signals on the Suburban loop track. This tower (a small 1-story structure located at the tunnel opening to the underground loop) contained a switch machine with a 11-lever Electo-Pneumatic machine operating 8 dwarf Signals, 4 double-slip switches, 4 pairs of movable frogs, and 28 single switches.
Tower 3 was located at the far end of the yard just beyond the Fort Point Channel bridges. This Tower also had 11 levers that controlled the approaches to the Loop Track as well as the main line approach tracks to South Station for the New Haven & New York Central Railroads.
|Semaphores mounted on Signal Bridge|
In the beginning of operations, oil lamps were used for the signal lights at the station with the exception of one signal bridge, where electric lights were tried on an experimental basis. Why experimental you ask ? because railroads were very conservative operations and something like electric lighting was still young in the minds of many in the railroading business. The "experiment" was likely as pretty good experiment in that by 1904, 182 electric signal lamps were in-use at the terminal.
|Model Board with position Indicators|
As built, the Tower was directed by a Interlocking Supervisor who, in-turn had an operating force consisting of: a directing dispatcher and his assistant, a telephone attendant, a telegraph operator, and 3 levermen all working on a 2-day shift. During the nighttime, the operating crew consisted of a dispatcher and 2 levermen. Mind you that on a typical operating day the huge amount of traffic generated approximately 28,450 lever movements per day.
The arrangement of trackage allowed for one side to be used for incoming trains while the other side would handle outgoing trains. The weakest part of entire interlocking was the 4 sets of crossover tracks in the middle that allowed trains to access any station track from any direction. If a locomotive or worse, and entire train derailed in the middle of any of the crossovers, it could put anywhere from 50%-100% of the entire interlocking out of service until the tracks were cleared and declared fit for use.
|Semaphore Models above machine|
After the Boston Terminal Company (The operators of South Station) came out of bankruptcy in 1952 the operating plant consisted of 17 out of the 28 original platform tracks, the Loop track & half of the inside yard tracks either removed or out of service. The biggest blow to come to the terminal was when Commuter service was terminated on the New Haven in July 1959. These trains alone, made-up one-half of the volume of South Station. At this point, tracks 1-9 were declared out of service due to lack of trains to fill them. After 1959, Tower 2 was removed, never actually used in active service. The U.S. Post office removed tracks 21 - 28 in order to expand their operation & facilities. The power plant & gas works were declared obsolete and demolished during this period.
By the 1970s, only tracks 8 - 17 were in-service. In 1974, the Railway Express building was demolished & the express tracks removed. The U.S. Post office built several new buildings which permanently removed tracks 18-21. The New Haven (now Penn Central) removed 2 of the 6 approach tracks as they were no longer in-use.
By 1977, only 57 of the Interlocking machine's original 143 levers were working, controlling 55 signals, 17 single switches, 18 double-slip switches, and 9 movable frogs. 55 of the unused levers were removed from the machine while others were reserved as spares.
In addition to the original machine, a 7-lever Electro-Pneumatic machine built in 1939 was added to the tower in the late 1970s when Tower S.S. 237 (Tower 3) was demolished and the switches were now remotely controlled from Tower 1. A new model board was installed next to the original one inside the tower to indicate track occupancy. By the end of the 1970s, the Interlocking plant served 144 scheduled trains in & out of South Station. This traffic included 24 amtrak trains & 130 Commuter trains.
When South Station was completely rebuilt starting in 1984, Tower 1 was finally demolished as the station, platforms, and trackage were completely rebuilt by the MBTA. Today, there are 13 platform tracks currently in operation with the interlocking controlled remotely from inside South Station in the CETC (Centralized Electric & Traffic Control) Center.
Images of Tower 1 from the late 1970s outside & inside.
|Interlocking Machine looking North|
|Interlocking Machine looking South|
|Indicators @ North end of Tower|
|Relay Cabinet on 1st floor|
|Signal bridges on approach|