Tower 1 - A History

Tower 1 @ South Station Terminal

When South Station was designed & built back in 1897 the purpose of the terminal was to consolidate 5 different railroads with their traffic, into 1 single facility. To do this required an enormous amount of planning & engineering that needs to take railroad traffic from 10 individual approach tracks and guide them to any one of the 28 station platforms safely & quickly. To accomplish this task, great care was taken to design the station's interlocking plant to be able to handle the enormous amount of trains that were expected to arrive & depart every day. For a train to be able to reach the proper track when arriving or departing, a bewildering system of switches was built in order to move trains to the proper track without crashing into each other.

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

If a Mechanical  Interlocking plant were to be installed at South Station, it would require a tower 160ft long with 360 levers & 45 feet of space on each side of the tower to accommodate the load out piping that ran from the tower to each switch - all 130 of them. Space being a premium where the station was being built, another solution needed to be found.

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.

Tower 1

Tower 1 is a 3-story brick structure with a Slate roof and sits in the middle of the track complex. On the first floor of the structure are switch relay cases. The second floor contains the Tower's generating system that converts AC power to DC for use by the machine. The third floor is encircled by windows allowaing a clear view of all track approaches. The Switch machine itself has 143 levers - 130 operational & 13 spares. These in-turn operated 91 High Home Signals, 36 Cautionary Signals, 21 Dwarf Signals, 31 Double Slip Switches, 31 Movable frogs, & 40 Single turnouts. All together this means the switch machine can handle 148 Signals & the equivalent of 233 switches. The levers that control Switches are painted blue, the signal levers are painted red, and the spare levers are painted white.

Interlocking levers & locking bed

Above the machine is a model board operated by minuture mechanical linkages which are mounted on the rear of the board & driven from connections to the switch levers. There are miniature working Semaphores at the top of the model board. A leverman could move a lever, see the actual Semaphore in the Yard change position, and then see the arm of the model Semaphore move to reflect the new position. At the North end (Head end) of the Tower are other indicators which show signal positions.

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

To provide information to the locomotive engineers, the yard used 9 signal bridges to hold the various Semaphores which in-turn displayed if the switches were properly aligned by the Tower.  The Semaphore blades & lights adopted were in accordance with the system used by the New Haven Railroad. The New Haven having recently adopted a standardized system of colors used by the Signaling system: Red for Stop, Yellow for Caution, and Green for Proceed. (replacing white for proceed). This color code was adopted for use in the station's signals. Air cylinders were usd to move the signal arms  & switches with Magnetic valves controlled by wires which extended to contacts of the interlocking machine. Two Ingersoll-Sargent air compressors in the power plant supplied the compressed  air for the operation. One compressor always being held in-reserve as a relay to the other. The air mains were duplicated so that failure in one line with not prevent immediate operation of the switches and signals through the other line.

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

Rail circuits on the station tracks controlled the cautionary signals governing the approach of trains to the station and indicators within Tower 1. A set of 28 indicators (one for each station track) were activated by the rail circuits & showed the presence or absence of trains on the station tracks.

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

The Interlocking plant saw little change from the time it was built until the 1929-1931 upgrade of the station. After the train shed was removed, the number of active use tracks were reduced from 28 to 17 with the decline of traffic during the great depression. In 1934, the new U.S. Post Office was constructed over tracks 21 - 28 which were being used as storage tracks by this time. Also by 1934, with tracks 18-28 not in active service, the Semaphores were removed as they were no longer needed and the Signal bridges were cut back in order to reflect the cutback in service. Signal Bridges 1 & 3 were removed completely, Signal bridge 2 was reduced in size, & Signal bridge 5 was cut in-half.

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

ALTERNATING CURRENT-DIRECT CURRENT' 
MOTOR GENERATOR SET AND POWER SUPPLY 
CONTROLS, SECOND FLOOR

Signal bridges on approach