• Basic signaling

  • For topics on Class I and II passenger and freight operations more general in nature and not specifically related to a specific railroad with its own forum.
For topics on Class I and II passenger and freight operations more general in nature and not specifically related to a specific railroad with its own forum.

Moderator: Jeff Smith

  by Tdub
This is only my second post as a new subscriber and as a relatively new railfan. So much info here, great forum. So, I know I will never really understand wayside signaling, I'll leave that up to the professionals. I just have a couple basic questions so please pardon my ignorance. Living in Sandusky, Ohio I have the good fortune to view the action on the NS mainline. There is a very busy diamond for a north/south line that not only crosses the main, but also has the ability to access the main, a few control points and a draw bridge. Signals everywhere! Here I go. When driving over one of the many grade crossings, I see a signal bridge that is almost always red. I think it protects the diamond but not sure. Maybe it is a block signal. I have no idea. But, when I see a yellow over red or green overhead I know a train is coming. My first question. I assume a far off dispatcher is controlling that signal, Right? Or could it be a block signal? Are block signals different than dispatcher controlled? I also notice that once the train passes under it, it goes red. Does the dispatcher change that to red as well or does the presence of a train control that? Last question, there are many hand thrown switches nearby since there used to be a yard just off the main. If one of those switches was in the wrong position, would that trigger a red since there is no way a crew member could see a switch in the wrong position? I know these are basic questions so again I am a novice at this so I apologize. This forum is a great resource and I appreciate any and all input.

Sandusky, Ohio
  by ExCon90
I think the best way to explain this is to start with presence detection--where is there a train? The NS main line has automatic signaling, so we can stick to that for now. A "track circuit" consists of a low-voltage current in one of the two rails and returning in the other, being connected at both ends of a section by a wire passing through a current source at one end of the section and a relay at the other to detect that the circuit is complete. If a train or engine is in the section the current will be diverted--"shunted"--through the wheels and axles of the cars, preventing it from reaching the relay, resulting in a binary function; i.e., 1 = circuit complete, no train present, or 0 = circuit not complete, train--or something--present in that section. This constitutes a building block on which all information pertaining to the presence of trains is based.

Where automatic signaling is in effect, as here, signals fall into two categories: fully automatic, and controlled. A fully automatic signal automatically displays whatever indication is appropriate to track occupancy ahead, as determined by the track circuits. Controlled signals are nowadays electronically controlled by a dispatcher. Historically, there were "block operators" at towers along the line, but today all significant main lines are under the control of a dispatcher who directs movement over hundreds of miles of route--somebody on here knows where the dispatcher for that part of NS is sitting. When the dispatcher desires to line a route for an approaching train, he "clears" the signal at the entrance to the appropriate route--the switches line up automatically and the signal then displays the most favorable indication consistent with that route and the distance from any preceding train.

As to the second question, when a train passes a signal the front wheels and axle immediately shunt the circuit, automatically causing the signal to change to red.

To the third question, hand-thrown switches are normally wired in with the track circuits so that when a switch is unlocked by hand prior to throwing it the track circuit is shunted just as though a train were there. There are rules and special instructions in effect prescribing exactly how this is to be done to avoid conflicting with a movement which has already been set up.

On most railroads a fully automatic signal has a number plate on the mast, usually 3 to 5 numerals and sometimes a capital letter, while the absence of a number plate identifies a "controlled home signal" controlled by the dispatcher. Any yellow or green on an automatic signal simply indicates the conditions ahead, but on a controlled signal indicates that the dispatcher is expecting a train. (A totally red controlled signal, however, may also mean that a train is expected but the needed route is at that moment occupied by another movement.)

I think the signal rules in effect on that part of NS (although they may have changed following the split of Conrail) are those of the Northeast Operating Rules Advisory Committee (NORAC). One website that shows them is
http://www.hubdiv.org/docs/signaling/NORAC.pdf" onclick="window.open(this.href);return false;
with the signal aspects and indications beginning on page 76. They can help visualize what's happening on site.
  by Tdub

Thanks so much for your reply. Great information. You really answered my questions. Thanks again.