Cab Signaling and how it works

News or chat about railroad info that pertains to the entire United States, another state, or country.
User avatar
Railfan_Julian8849
Railroadfan...fan
Posts: 10
Joined: Sun Nov 28, 2021 6:53 pm
Location: Chesterton, Indiana
Contact:

Cab Signaling and how it works

Unread post by Railfan_Julian8849 »

So I have seen how on Norfolk Southern on a lot of trains they have to add an NS leader because of cab signaling. Well how exactly does cab signaling work and does it work like PTC?
Resident GE foamer :lol:
Chesterton area railfan who frequently can be seen along NS's Chicago Line and CSX's Grand Rapids Subdivision

User avatar
Standard Railfan
Railroadfan...fan
Posts: 1696
Joined: Wed Jan 18, 2012 7:25 pm
Location: Marquette, MI

Re: Cab Signaling and how it works

Unread post by Standard Railfan »


User avatar
Tom49801
Bangor Webcam Master
Posts: 16157
Joined: Sat Oct 06, 2018 9:17 am
Location: Prince Frederick, Maryland

Re: Cab Signaling and how it works

Unread post by Tom49801 »

Railfan_Julian8849 wrote:
Tue Nov 22, 2022 8:58 am
So I have seen how on Norfolk Southern on a lot of trains they have to add an NS leader because of cab signaling. Well how exactly does cab signaling work and does it work like PTC?
Cab code signaling involves placing AC voltage pulses onto the rails that are received by a pick-up coil (antenna) that is located in front of the leading wheels of an engine & about 8 inches above the rails. The pick-up coils look like a horizontal bar & the AC voltage pulses (we say “coded” AC or “Code Rate”) is fed to the on-board cab signal equipment that calculates the code rate (how many AC pulses per minute) & if they are within the allowable tolerance set for each code rate.

On the engineer’s console, or, a separate device in the cab, a signal will be displayed to indicate what can be expected at the next wayside signal up ahead. This is especially helpful for curves, inclimate weather, fog, & should the signal up ahead were to change before it’s within visual range. If the cab signal changed to a less favorable signal, the engineer had about 20 seconds to start reducing his speed & comply or the train would automatically apply the brakes. PTC won’t be replacing cab code systems. The wayside intermediate signals along Amtrak’s Northeast Corridor are starting to be removed & the engineer will rely on the on-board cab signals.

The track routing & track circuit occupancy determines which code rate (AC pulses per minute) the signal system equipment is to transmit onto the rails. The PRR had chosen 100Hz AC voltage for their signal system, since 25Hz is used for powering their electric locomotives. This was to help prevent the 25Hz AC currents from influencing the signal system. Amtrak’s NEC still uses 25Hz AC and a few years ago in Connecticut 60Hz power was installed when a portion of the railroad became electrified for the 1st time.

Code Follower relays are used for coding the 100Hz AC voltage by the relay’s contacts opening & closing at the prescribed code rate. The “coded AC voltage” is then coupled to a transformer and placed onto the rails via underground wires leaving the signal bungalow or wayside case.

The different code rates are “pulses per minute” (PPM) and the original on-board cab signal could display 4 different signals: Restricting (no coded pulses received due to track occupancy in the block ahead or a broken rail), Approach (75 PPM), Approach Medium (120 PPM) & Clear (180 PPM).

A few years ago, Amtrak’s NEC upgraded from 4 to 8 on-board cab signal aspects by adding 250Hz as a 2nd AC frequency placed onto the rails & a new 270 PPM code rate. In addition to the original 4 PPM rates using 100Hz only, the following cab code rates were introduced for the high speed crossovers & traffic congestion in the New York area:

100Hz & 250Hz @ 120 PPM rate for High Speed crossovers: 80 mph for Acela Trains & 60mph for the other type passenger trains.

100Hz @ 270 PPM rate: Cab code 60 mph (used in high volume traffic areas).

100Hz & 250Hz @ 270 PPM rate: Clear 100 mph.

100Hz & 250Hz @ 180 PPM rate: Clear 150 mph.

User avatar
SD80MAC
Ingersoll's Mr. Michigan
Posts: 9922
Joined: Thu Mar 10, 2005 4:59 pm
Location: Grand Rapids

Re: Cab Signaling and how it works

Unread post by SD80MAC »

PTC has actually negated the need for cab signals on several lines. Union Pacific, I believe, as of this year has shut off all of the former CNW cab signal systems on the old CNW, which has lead to some interesting leaders on lines which could only ever have UP cab signal equipped leaders before. I believe NS is in the process of doing the same thing on the old PRR?
"Remember, 4 mph is a couple, 5's a collision!"
http://flickriver.com/photos/conrail680 ... teresting/
Image

EWRice
Railroadfan...fan
Posts: 444
Joined: Fri May 10, 2013 7:07 pm
Location: Muskegon, MI

Re: Cab Signaling and how it works

Unread post by EWRice »

I highly recommend that you look for the book "Railroad Signaling" by Brian Solomon. It is a good introduction to the history of signaling and its evolution up to PTC. It is a very good start to understanding how various types of signaling, rules and track circuits work.

User avatar
Saturnalia
Authority on Cat
Posts: 15018
Joined: Wed Sep 28, 2011 7:54 pm
Location: Michigan City, IN
Contact:

Re: Cab Signaling and how it works

Unread post by Saturnalia »

SD80MAC wrote:
Tue Nov 22, 2022 9:06 pm
PTC has actually negated the need for cab signals on several lines. Union Pacific, I believe, as of this year has shut off all of the former CNW cab signal systems on the old CNW, which has lead to some interesting leaders on lines which could only ever have UP cab signal equipped leaders before. I believe NS is in the process of doing the same thing on the old PRR?
Similarly, BNSF recently retired the old Santa Fe ATC on its Transcon, for similar reasons. PTC isn’t “vital” but it does basically what cab signals and ATC did, so the FRA has allowed the discontinuances.

With PTC effectively displaying the signal indications by showing the prescribed speeds, the need for another method of displaying the actual indication is superfluous.

I could see a future where in “PTC 2.0” all intermediates are gone and absolutes remain as two-aspect “stop or go” signals only, with PTC relaying everything via speeds rather than an actual aspect.

There’s no need to begin slowing to 30 MPH passing an approach, for instance, unless your trains stopping distance actually requires it. PTC is able to handle each train according to its make-up, rather than a one-size fits all strategy based on the worst case scenario.

In fact, I would argue that many of PTC’s greatest annoyances are based on it relying on the signal system’s aspects. Since PTC itself acts on a worst-case braking scenario, it compounds signal aspects which are already designed for the worst case scenario - this compounding leads to a lot of unnecessary early slow downs and speed restrictions.

In my prior example, PTC will hold a train to 30 at an Approach rather than beginning the reduction to 30 at the signal. Double restrictive when you add PTC to signals. But if you nuked the fixed wayside signal, PTC can simply enforce the stop or restricting after it and adjust the braking distance accordingly, potentially allowing trains to continue faster longer.

But instead, right now, you get both PTC’s conservatism on top of the signal system’s conservatism. That means slower trains.
Thornapple River Rail Series - YouTube
Safety today is your investment for tomorrow

Post Reply