block signals

[CrazyDiamond]

Great response 1435mm! :-)

[Anonymous]

1.  Combinations of red, yellow, and green (multiple heads, flashing, etc.) exist to modify the basic aspects of red, yellow, and green.  Each railway has its own practice and restating it here is not feasible.  However, it's key that there are two basic practices of signaling, SPEED SIGNALING where the signal aspect indicates a maximum speed and routing is inferred, and ROUTE SIGNALING where the signal aspect indicates the route that will be taken and the maximum speeds are shown in the employee timetable on a location-by-location basis.  Most railways follow one or the other practice.  Search on Google for the railway you're interested in and the words "signal indications" and you'll find most or all of them.

2.  Your question regarding when signals light is easily understood by understanding track circuits and approach lighting, and operating rules.  When a train passes a block signal governing its direction the signal lights in the opposite direction (1) because the track circuit is closed in the facing block, and (2) so the train knows where the block it has now entered begins and ends.  This is important for reverse moves.  It has nothing to do with opposing traffic as there couldn't be a train in that block anyway (unless it has been disregarding signal indications until that point, it would start observing them now?).  Most railways have long ago converted to approach lighting (the signal doesn't light until the track circuit facing it or the one it protects) are occupied in order to save on bulb life, save battery life (if utility power is down), and to minimize vandalism.

3.  Many railways have increased block signal spacing to account for longer and heavier trains with longer stopping distances. 

4.  There are no block signals being phased out in the same fashion as telephone poles.  Block signals have been deactivated and removed on *many* lines that have been downgraded due to loss of traffic (particularly passenger traffic) but they are not being phased out in preference to something better such as Communications-Based Train Control or Positive Train Control, at least not yet (the IDOT project is still a testbed). 

5.  Block signals are actually controlled locally by the internal logic of the Automatic Block System or Manual Interlocking (Automatic Interlockings and ABS without CTC overlay are entirely automatic; no one controls them).  Train dispatchers and control operators select PRECEDENCE and ORDER of traffic for CTC systems and Manual Interlockings, and the ABS system executes these commands within the limits of the safety and conflict logic that's built in. 

6.  There is no longer any practical limit on the distance between the control operator or dispatcher and the field installation.  The very first CTC systems were "hard-wire" and had about a 50-mile practical limit.  Early coded installations allowed up to 150-mile limits and you saw a lot of CTC offices located in division offices or in the approximate center of an installation (e.g., the Rio Grande CTC offices at Green River, Utah; Hot Sulphur Springs, Colo.; Grand Junction, Colo.).  Later coded installations removed all practical limitations (though you have to account for the latency of the transmission system).  By the 1960s railways were consolidating outlying offices and this process continued from divisional consolidation to regional consolidation to system consolidation.  Today BNSF, UP, CSX, KCS, and CPR have centralized dispatching systems that can control the entire system, though for purposes of coordination with other railways BNSF and UP have slightly decentralized with small offices that control major traffic hubs such as Kansas City, Chicago, Los Angeles, and Houston.  If you wanted, you could control the entire railway from Bangalore, India.  But for practical reasons that have nothing to do with technology I do not expect to see such a thing anytime soon.

S. Hadid

[CrazyDiamond]

I sent you an offline email.....

[Anonymous]

Good morning--

I was just curious if anybody out there is familiar with what the meanings of all the various signal configurations are for the CTC block signals on the Class Is are.  For example:  Obviously red, yellow and green are like auto traffic signals.  I've seen some blocks with a flashing red or flashing yellow.  What do they mean?  For that matter, has anyone out there seen a flashing green?  Second, this may sound somewhat strange to ask, but are these blocks only for the benefit/purpose of alerting immediately oncoming trains?  Let's say, for example, you have an eastbound train.  Obviously, the block facing to the west is going to go on to alert that oncoming eastbound to go, stop, or slow down.  My experience has been that the block signal facing east does not actually go on until after the eastbound train has passed it (which seems kind of ridiculous to me, since any train coming from the opposite direction would see the headlights before they'd see the block signal anyway).  If the piece of track is heavily trafficked, why wouldn't it make sense for the eastbound block to go red BEFORE the actual eastbound passes it, to alert any potential westbound train that something will come soon?  Third, is there any indication that block signals are now fewer and farther between, and/or that the big roads are gradually phasing them out in certain locations, in much the same way as the telegraph poles have been/are?  Finally, since (I guess) these blocks are activated/deactivated from somebody in a single control tower, is there any rule about how far away the controllers are?  Can (and do) the controllers in Kansas City (for example) control the block lights going into LA? If we ever get a coast-to-coast Class One merger, would that system still hold?

Riprap

Riprap