VIDEOS: US&S CTC Machine Operation in Amtrak's THORN Tower

The Union Switch and Signal CTC style unit lever interlocking machine was developed in the 1930's to serve as a user interface for relay based interlockings under remote or local operation.   Amtrak's THORN tower was built by the Pennsylvania Railroad in 1937 as part of its Philadelphia to Harrisburg electrification project and sought to consolidate a number of manned interlocking stations into a single tower.  83 years later THORN tower remains in operation, although with a significantly reduced territory and gradually approaching plans for retirement.  I was able to obtain some video clips of the THORN's CTC machine in order to preserve some degree of its live operation for posterity and this post will use those videos to expand upon my existing coverage of the tower interior and THORN interlocking.  If you are unfamiliar with THORN interlocking I would encourage you to visit those pages first.

This aborted take provides a static view of the CTC interface as a SEPTA local has come to a stop or near stop within interlocking limits as it crosses from track #4 to track #1 to finish its run and change directions at the eastbound station platform.  We see the upper row with switch levers 41, 47, 49, 53 and 57 set reverse as indicated by the unit levers pointed at the "R" position and the associated yellow illuminated indicator lamps.  A red lamp above all switches aside from 43 and 61 indicates that those switches are currently locked for this particular extreme crossover movement. The lower row contains the signal and switch heater unit levers.  Currently all signals are locked in Stop status due to the extreme crossover movement.  On the model board a yellow track occupancy light (TOL) is illuminated by the 57 switch, indicating the approximate location of the SEPTA local.

Here the Train Director at THORN tower sets the route for the above SEPTA local.  The operator toggles the 57, 53, 49, 47 and 41 in the order the train movement would encounter them, although this is not required. After each toggle the operator presses the "code" button on the bottom row below each unit lever.  This transmits the state of the unit lever to the interlocking logic or code system similar to an "Ok" or "Apply" button on a computer GUI.  THORN interlocking is under direct wire control from the CTC machine and I was told that this makes it necessary to press the code button for each unit lever movement as otherwise the actions would take place immediately after the switch toggle, which was not considered desirable.  I believe a remote plant just needs one code button press to send all commands, however a signal command is always selected after the switch commands to avoid race conditions which might result in a mis-routing.

US&S 504B Code relay equipment.

After the code button is pressed the relays in the basement send a command to the US&S Style CP air-valve that then throws the Model A-5 pneumatic point machines.  Note at time 0:05 the sound of 49 switch throwing. As each switch throws the red switch lock light illuminates which the points throw and when the throw is complete the green "Normal" lamp extinguishes and the yellow "Reverse lamp is lit.  As no routes are set. the red lock light goes out.  The Train Director waits for the points to line and lock into the desired position, at which point he requests the 58L signal to display over the established route.  The interlocking logic determined the route is valid and red lock lamps then illuminate over every switch locked out by the new route, which in this case is all of them aside from 43 and 61.  When the interlocking logic has confirmed all the proper conditions are met, the signal displays in the field and the yellow 58L "signal displayed" lamp illuminates. Note, the red lamps over the signal unit levers indicate neither the L or R signals are displayed, but they sometimes go out so they might also represent some sort of locking condition.

 

58L signal displaying Restricting

Here we see a rogue TOL on Track #1 in the vicinity of the Whitford station.  This was intermittently caused by the actions of a signal maintainer.  The TOL on track #2 was a known fault, but may have been in the process of being remedied, however because track #2 is currently used infrequently it is not uncommon for track circuit faults to be left in place.  The Train Director presses the code button under a catenary circuit controller to trigger the CTC machine to send a supercilious command that will trigger the remote logic to reply with an updated status. The sounds are from relays connected to the US&S 504 code system. While the unit levers for THORN interlocking are wired directly to the relay logic in the basement, the UI elements for remote interlockings are connected to telecom grade relays in the CTC machine that store, forward and receive state from the 504 code system.  This works via telephone style code pulses on a shared "code line".  Codes have a station ID and a command or a status.  In the attempt to verify the status of a intermittent TOL, you can hear the CTC remote coding system going back and forth with status updates.  Note how lamps in the process of being updated briefly extinguish.

 In this sequence an eastbound Amtrak Keystone Train on track #1 travels through the CALN remote interlocking, traverses the 1-2 miles between CALN and THORN and then enters and passes through THORN interlocking all on a straight route with clear signals.  Note the relay sounds as the TOL on the approach blocks, CALN interlocking and the intermediate block change state.  The multiple approach block TOL's are holdovers from obsolete configurations of the interlocking that had a signal protecting some industrial sidings that was removed decades ago.  At 0:32 the train passes into the cab signal cut block/  This is an extra block that supports locomotive cab signals changing to Restricting in advance of a stop signal.  From this point no lamp changes generate audible relay noise as the lamps are cirectly wired into the shelf relay interlocking logic in the basement of THORN.  

The twin blue lamps on track #2 west of thorn represent current of traffic (and thus the reason for the name Centralized Traffic Control).  Although designed for bi-directional operation, the PRR was not a proponent and when installed in 1937 this CTC panel had almost no bi-directional trackage, with some segments only getting it years later as the physical plant was consolidated. 

 As the Keystone train enters THORN interlocking at 1:10, the 44R unit lever status changes from displayed to stop at 1:13.  A few seconds later the train passes by outside the tower and then proceeds to pass through the interlocking.  A major challenge of traditional towers such as this one is keeping track of train movements as there is no automatic train identification or labeling.  Tower operators used to pass "next train" information from one to another.  As towers became isolated islands they would rely on the dispatcher or data feeds via computer from the dispatching system.

Here we watch as a westbound Amtrak Keystone train transits DOWNS interlocking just before a station stop at Downingtown.  The TOL's begin on track #4 east of the 76R signal, then also on track #4 within the interlocking limits before the TOL east of 76R goes out.  As the train exits the interlocking limits past the 76L signal the TOL west of DOWNS illuminates before the TOL within DOWNS goes out.  Note the 70R unit lever is displayed at DOWNS on track #1 for an eastbound movement while the 76R signal stays displayed for following westbound movements.  This is because the 76R was set to "fleet" by pressing the button directly below the signal unit lever.  The 58L signal at THORN is also displayed.  Cancelling a signal involves setting the unit lever back to the central neutral position and hitting the code button.  The red lamp above the signal will then flash until the timer runs down (if the signals are not fitted with approach locking).

In the next series of thee videos we will watch a westbound Amtrak Keystone pass through GLEN interlocking, DOWNS interlocking and then THORN interlocking in sequence.  Due to a maintenance hold the train was moving slowly in the approach block of the 104L signal at GLEN interlocking.  The first video begins immediately after the command to display the signal was initiated.  The 104L status lamp changes to "displayed" at 0:14. 

The train takes a while to accelerate and the video pans to the more modern FRAZER unit lever panel that was installed in 1994 when FRAZER interlocking was placed in service.  One can see the 1E and 4W signals are both displayed and set to fleet with an illuminated yellow lamp (fleeted) over a green lamp (displayed).  The red lamps next to the yellow TOL's are manually set as an indication of de-energized overhead catenary.  Back to the CTC machine, the train enters GLEN interlocking at 1:55, causing 104L to change from displayed to stop.

Here the train is passing through DOWNS interlocking, but unlike last time the 76L signal is not set to fleet. At 0:22 the CTC UI gets a status update flipping the TOL on DOWNS track #4 to occupied and the 76L status from displayed to stop. 

 

 
Finally we have a clip of the westbound Keystone train passing through THORN interlocking on track #1.  Video starts a bit late after it has already dropped the 58R signal.  At 0:09 the train crosses into a pocket track circuit that exists due to now removed 59 switch on track #1 that was protected by an also removed 58L signal.  This cues me to switch to the window as the train then passes in front of the tower and exits the interlocking. When the track #4 pocket circuit is vacated the locked lamp for the 61 switch (set normal) goes out.  Also note the 44R signal displayed for an eastbound movement. 

 

Well I hope you found this interesting.  Unfortunately this ability for everyone to have an HD video camera in their pocket did not exist in 2005 so I was unable to obtain video of every possibly corner case regarding specific CTC machine.  Moreover, these machines, while once the produce of a standardized production process, have been dramatically altered over the years by the dedicated efforts of the signal maintainer who lives in their own domain in the tower basement.  Therefore do not take what you see here as a definitive guide to this technology. Operation may vary by time period and by railroad.  That being said please feel free to note anything I may have gotten wrong in the comments.