• Signals

  • General discussion about railroad operations, related facilities, maps, and other resources.
General discussion about railroad operations, related facilities, maps, and other resources.

Moderator: Robert Paniagua

  by MetalCowboy
Can anyone explain how exactly signals work? I have been witness to signals that stay continously red unless a train is actually withing a mile or two and others that shut off after the train passes. Also some turn yellow or green when a train is passing, but do not do the same all the time. Why?

  by eddiebear
I don't know all there is to know about signals, but signals that light up when a train nears the block are known as Approach Lighted. The Boston and Maine generally used this type of signal. This type is also used on the CSX Boston Line, in locations where there are wayside signals. The old New Haven RR signalled routes generally had signals which were lit continuously.
The schools of thought were the Approach Lit signals and related equipment lasted longer because they were in use only which trains were in the block. However, Approach Lit signals require additional equipment, hence additional installation cost, to turn the signals on and off.
  by steemtrayn
MetalCowboy wrote:Can anyone explain how exactly signals work? I have been witness to signals that stay continously red unless a train is actually withing a mile or two and others that shut off after the train passes. Also some turn yellow or green when a train is passing, but do not do the same all the time. Why?

Signals at interlockings are at a "stop" indication until the operator or dispatcher prepares a route for a train to run through; this usually happens when the train gets close.. Signals in automatic territory where traffic runs in both directions are red for the direction that is not established by the dispatcher.

Can't help ya with the green and yellows, I always figgered they turn red when the train passed.

  by CRail
Different colors give different indications. if you got a yellow over green, its approach medium. Green over red is clear, yellow over red is clear, and so on. there are ,many indications but those are some of the more common ones.

  by CSX Conductor
yellow over red is clear? since when? lol

  by themallard
CSX Conductor wrote:yellow over red is clear? since when? lol
Would that be approach limited @ 30 mph and yellow over green be approach medium @ 45?

  by CRail
Yellow over green is approach medium. If the green light is blinking, then its approach limited. There is a typo there, yellow over red is approach, and i believe the speed is 30. Limited speed is 40 for freight and 45 for pass...

  by MBTA F40PH-2C 1050
red over flashing yellow = medium approach 30
red red over green = slow clear
red over yellow = restricting
red with # plate = stop and proceed

  by 2nd trick op
Interestingly, at the controlled siding in my home town (NS/CP Sunbury Line, formerly D&H, formerly PC, formerly PRR), it's the home signals which are approach lighted. The distant sgnals are lighted constantly.
  by fglk
Last edited by fglk on Thu Aug 19, 2004 7:17 pm, edited 1 time in total.

  by clearblock

Your information is generally correct about CTC and train signaling but you are misinformed about crossing protection.

There is no use of the FRED train telemetry equipment to control grade crossing signals.

Grade crossing protection is presently track circuit based with three general types in use.

The traditional crossing circuit uses the dc current on the rails the same as for the CTC signaling. Insulated joints provide an "island" circuit on the crossing and an "approach" circuit on each side of the crossing. The length of the approach circuit is determined by the maximum train speed.
When a train is on the approach circuit the track relay for that circuit drops and the crossing is activated and remains activated until the train has passed the island circuit. When the island circuit is clear the relays reset and the crossing protection clears. There are 2 problems with this system.

A slow moving train or a train stopped in the approach circuit will cause the gates to go down too soon. If a train doing switching passes over the crossing and clears the island the gates will go up. If the train then makes a reverse move back over the crossing the gates will not go back down until the train is on the island circuit again.

The next type is the Motion Detector. It uses a high frequency AC signal
for the approach circuits and either a DC or different frequency AC signal for the island circuit. The Motion Detector senses a disturbance in the AC signal when a moving train is on the approach circuit and activates the crossing protection. If the train stops short of the crossing, the Motion Detector can sense this and will clear the crossing after a slight time delay to be sure the train is really stopped. When the train moves again, it will reactivate. This solves the problem of stopped trains or stop and reverse moves causing improper activation.

The newest type is the "Predictor". It is similar to the Motion Detector but it has a computer that measures the speed of the approaching train by measuring the phase shift on the AC track signal. This is the same "doppler effect' that is used in police speed radar but it is done by a signal on the rail not a radar signal. The Predictor has all the features of the Motion Detector but it determines when to activate the crossing protection based on the speed of the train.

Any radio based system as you described would not meet FRA "fail safe" requirements beacuse any failure to receive the radio signal would mean that there would be no protection. The present systems are all set to keep the protection off only if there is no interruption or disturbance of the constant AC or DC current on the rails. This is why a broken bond wire, road salt on the ballast or anything abnormal causes the crossing protection to activate and stay on until fixed.

Your statement about voltage on one rail being conducted to the other by the train to operate the circuit is also not exactly correct. Under the fail safe principle, there is a constant current circulating out one rail and back the other through the track relay or the AC sensing circuit in a Motion Detector or Predictor. This constant current keeps the track relay or sensor energized and the crossing protection off. If power fails, or something breaks or the presence of a train axle shorting the rails disturbs the current then the track relay drops and the crossing is activated.

There have been some cases where a radio control is used to manually operate crossing protection in an industrial or switching situation. This simply eliminates the need for a crew memeber to get off to push the buttons at the crossing box. This radio control is separate from the FRED or other on board system and does not replace the normal track circuits.

  by SteelWheels21
Here are some signals we use out here on UP:

Green - Proceed
Flashing Yellow - Advance Approach
Yellow - Approach
Yellow Over Flashing Green - Approach Clear Sixty
Yellow Over Green - Approach Clear Fifty
Yellow Over Lunar (white) - Approach Restricting
Yellow Over Yellow - Approach Diverging
Red (no number plate) - Stop
Red (number plate) - Stop and Proceed
Flashing Red or Lunar (white) - Restricting
Red over Flashing Green - Diverging Clear Limited
Red over Green - Diverging Clear
Red over Flashing Yellow - Diverging Advance Approach
Red over Yellow - Diverging Approach
Red over Yellow over Yellow - Diverging Approach Diverging

Not to mention some variations on these for passenger trains, semaphores on the Phoenix service unit and some really funky looking signals at Lake St. Interlocking in Chicago.

  by Delta
There are few commonalities or hard and fast rules when it comes to railroad signal aspects and indications. Not only can signal aspects have different indications on different railroads, but I've seen the same signal aspect can have different indications on one railroad.

Without knowing the specifics of what type of signal territory and what type of signal is being observed along with that railroad's rules regarding signal apsects and indications and any modifications made to them by timetables or special instructions it is absolutely impossible to know what any given signal means or how it operates.
  by jg greenwood
Different railroads have different signals indications. On the IC here in the midwest, yellow over green: Approach Diverging. Let us exercise caution with our......generalization.

  by starionwolf
Thanks for the information about signals. I have heard about interlockings and switches.

When I was on the MetroTrain headed towards the Alexandria train station, I saw some red signals over the freight tracks. Cool!

I am more familiar with the MetroRail subway in Washington D.C. This system uses block signaling and automated train control. I don't know how similar the MetroRail system is with respect to the other signals.

Most signals are near the platforms of some stations.

The MetroRail uses red and white signals. A white lunar signal means "go" while a flashing lunar means diverging track ahead. Red over read means stop - no train can go into the protected "block" or section of the track. I've seen white signals can be between stations. Other signals are obviously at double crossovers and near divergent tracks.

Some signals can be manually cranked or block into different positions if the signals are not working correctly. I didn't realize that running a railroad was lots of work!