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  • How is the direction of travel set on CTC signals?

  • Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.
Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.

Moderators: mtuandrew, therock, Robert Paniagua

 #1380240  by MCL1981
 
How do the signals on a bi-directional double track CTC system like the Brunswick Line know what direction trains are expected to be traveling in? For example, in a stretch of several clear blocks, I would imagine the signal system isn't showing green in both directions at the same time, or is it? At an interlocking, where there isn't a train in the vicinity, how does it decide what direction gets the green? Is this something the displatcher sets remotely at the control points? Or does the system make assumptions based on actual train travel on the lines?
 #1380803  by Head-end View
 
And to clarify, no way that signals in both directions could be green at the same time (except for a malfunction causing a "false clear"). I'm sure the "system" cannot normally do that. If for example, the dispatcher cleared the northbound signals for the next train, the southbound signals would be red automatically.
 #1380805  by MattW
 
Head-end View wrote:And to clarify, no way that signals in both directions could be green at the same time (except for a malfunction) causing a "false clear". I'm sure the "system" cannot do that. If for example, the dispatcher cleared the northbound signals for the next train, the southbound signals would be red automatically.
For controlled signals, you're exactly right, but for intermediate signals, that's not necessarily true. The third picture in this post from Jersey Mike's excellent signals blog illustrates this concept well: http://position-light.blogspot.com/2014 ... shots.html" onclick="window.open(this.href);return false; It makes sense, that way the only control logic is to the controlled signals at each end of a stretch of track, the signals just behave based on what the "next" indication is rather than what the controlled signals show.
I have seen this kind of thing in real life myself. Signals between control points will show clear in both directions until a train approaches close enough to trigger the sequence of stop->approach->clear or whatever the proper sequence is. I'm sure it sounds dangerous, but if you think about it, a train can't approach except from a controlled signal, and even if a giant stork happened to drop a bouncing baby train directly on the tracks without passing a controlled signal, it would still be fine because the signals surrounding that block would drop to stop, the signals around those would drop to their respective aspects (approach, then maybe advance approach if necessary or approach medium if you're CSX), and the dispatcher would grumble obscenities and go stork hunting.