Demystifying Amtrak’s radio CTCSS tones in the Northeast

CTCSS is a technique that inserts specific audio tones into radio transmissions but in what way does Amtrak use them in the Philadelphia-New Jersey-New York area?

Dec 06, 2024

This article was previously published on October 15th on a Substack profile instead of in a Substack publication: https://substack.com/home/post/p-149988315. This republication correctly places the article in a publication which has better features.

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of in-band signaling that is used to reduce the annoyance of listening to other users on a shared two-way radio communication channel. It is sometimes referred to as tone squelch or PL for Private Line, a trademark of Motorola. It does this by adding a low frequency audio tone to the voice. Where more than one group of users is on the same radio frequency (called co-channel users), CTCSS circuitry mutes those users who are using a different CTCSS tone or no CTCSS.
CTCSS tone codes are sometimes referred to as sub-channels, but this is a misnomer because no additional radio channels are created. All users with different CTCSS tones on the same channel are still transmitting on the identical radio frequency, and their transmissions interfere with each other; however; the interference is masked under most conditions. Although it provides some protection against interference, CTCSS does not offer any security against interception or jamming, and receivers without CTCSS enabled will still hear all traffic.

The above excerpt is from the Wikipedia article "Continuous Tone-Coded Squelch System", which is released under the Creative Commons Attribution-Share-Alike License 4.0.

How does Amtrak use them on the Northeast Corridor? I’m actually not entirely sure as I have never been inside either of the dispatching offices to hear what dispatchers hear and how they work. A quick analysis shows that the radio transmissions with CTCSS tones all have phase shifts of the CTCSS tone (about 125 degrees) about 200 milliseconds before the end of the transmission. This can be used to silence the squelch tail (also called squelch crash) so strong transmissions don’t crash into a bunch of static as the receiving circuit loses its “excitement”. This is out of scope for this post and you can read more about it in the “Reverse CTCSS” section of the above Wikipedia article.

This closeup of an audio capture (amplified for better visibility) from the end of a dispatcher radio transmission shows that the phase is “reversed” (delayed) at the 56ms mark (0.056 seconds) – the next peak after the phase reversal comes 37.8% late compared to the period (3.66ms at 103.5Hz) putting the phase delay at 136 degrees, close to what the Wikipedia article describes as a typical value. The CTCSS tone remains active for 200ms before the transmitter turns off. At the end is the squelch tail which is random static, caused by reception of an extremely strong signal leaving the squelch open for an excessive period of time. The squelch algorithm requires time to “realize” the signal is now gone and this problem exists in purely analog systems as well.

I have a collection of radio samples (captured from central New Jersey) transmitted from the Philadelphia area all the way to the Queens/Bronx area in New York; there are several typical patterns but there isn’t 100% consistency. There are 2 tones in use:

NS 13 / PL 1A (103.5 Hz)
NS 23 / PL 4B (146.2 Hz)

PL stands for Private Line, the trademarked Motorola name for CTCSS tones

NS is “non-standard” and defined by one or more other manufacturers

This is a pure 103.5 Hz tone:

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0:00

-0:05

This is a pure 146.2 Hz tone:

1×

0:00

-0:05

Many speakers are incapable of properly playing these tones. Mobile phone and laptop speakers may be incapable of playing them, so you may need a proper set of desktop speakers or headphones to hear those tones or the tones in the following audio clips. Radio equipment also usually can’t play these tones and they likely filter out (at the microphone and on playback) frequencies below a certain threshold.

These are the rough classifications for the CTCSS tones on the NEC:

103.5 Hz – Dispatchers and dispatching office
146.2 Hz – Amtrak portable radios, Acela cabs, Amtrak defect detectors, Amtrak maintenance of way vehicles
Never – NJ Transit conductors, Amtrak locomotives except Acela cabs, NJ Transit trains, SEPTA trains

Yet… not every radio has these tones including specific dispatcher stations. All I can do is observe patterns and present samples. Thus, I present the samples.

Terms, in no specific order:

Dispatching office – where dispatchers sit and almost all routing decisions are made. There are two dispatching offices on the Northeast Corridor south end: the New York Dispatching Office and the Wilmington Dispatching Office. The Philadelphia Dispatching Office has been gone for about 15 years from what I researched. Historically, these were controlled by hyper-local “towers”. The “K Tower” is a local tower in the Washington DC area governing the immediate area around the station. The Philadelphia-Harrisburg Line is almost exclusively controlled by local towers.
As for radio communications, the two dispatching offices are on different radio channels. Dispatchers in each respective office can hear each other (or it may be muted by CTCSS tone detection) and seem to strategically avoid talking over one another on the radio.
CETC - The CETC system is Central Electric and Traffic Control. It is often incorrectly abbreviated “CTEC” in part because it is pronounced “c-tec” and you are likely to find the incorrect spelling as often as the correct one. The CETC system is a digital communication system for dispatchers and includes signal, track occupancy, electrical monitoring, switch control and likely voice communications. This system allows highly centralized dispatcher offices compared to the “old” system of individual towers. CETC sections are given numbers like “CETC-8” representing a section of territory. Not all CETC sections are referred by this scheme. Section A, Section B and PSCC, the three dispatcher sections near New York Penn Station, are likely all CETC systems as well.
PSCC – Penn Station Central Control is a part of the New York Dispatching Office. Sometimes, PSCC collectively refers to all the dispatchers of the NY office since they are all in the same room. PSCC territory covers all the underground portions around Penn Station to just outside the tunnels on the New Jersey and Queens sides, respectively.
Section A, Section B - Two or three different dispatcher territories around Penn Station. Section A flanks PSCC on both the New York (Queens-Bronx) and New Jersey (Newark-Secaucus) sides. Section B is only on the New Jersey side in the Newark area.
NYP Line – New York-Philadelphia Line (employee designation for this part of the Northeast Corridor)
PW Line – Philadelphia-Washington Line (employee designation for this part of the Northeast Corridor)
PH Line – Philadelphia-Harrisburg Line (employee designation for this part of the Keystone Corridor)
MoW / MW – Maintenance of Way
Defect detector – Device embedded in the track to detect certain defects on trains such as hot bearings and dragging equipment. Most defect detectors around the country speak on the same radio channel as trains and dispatchers use so crews can hear any problems. Amtrak’s defect detectors only speak on actual defect or for testing.
C&S – Communications and Signals, a department of Amtrak. C&S employees are addressed “C&S” over the radio.
ET – Electric Traction, a department of Amtrak that deals with the electric system
Absolute block – A section of track reserved for a specific train or other rail vehicle where no other equipment may run. This can be done automatically/transparently (so trains can’t point at each other and get stuck) or manually such as when following the diesel train tunnel procedure to ensure no other train can be in the tunnel with a diesel and for 5 minutes after while the fans run.
Rule 241 – Verbal permission from dispatcher to pass a stop signal. Usually this is given to MOW vehicles entering out-of-service track (the signaling system is employed to protect these tracks) but can also be given to trains to pass malfunctioning signals.
Form D – written form (communicated verbally and confirmed by read-back) to take a track out of service, to run equipment against normal track direction, to run equipment at altered movement speeds or to run equipment with special instructions such as a ”drop pantograph” order

Some clips may be too loud so you should consider lowering your volume before playing each one.

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