Block Signal System

[SW 1200]

I have been reading a book titled

Michigan Rail Disasters 1900-1940 .

Several of the accident summaries mention Block Signal Systems.

What is a Block Signal System? 

How does does a Block Signal System work?

What do they look like? 

Any help appreciated.

 Thanks

TonyM.

[Railway Man]

 A block system subdivides the main track of a railroad into blocks (contiguous segments) with fixed and permanent limits.  The entrance to each block is governed by a signal that provides information about track conditions ahead, such as whether the block is at that time occupied by another train, or has an open switch, etc.  The block system may be automatic or manual.  Manual block systems use written or verbal authorities to enter the block, or a staff or token system where physical possession of a unique artifact is required to enter the block (the token or staff is dropped off at the other end of the block for reuse).  Automatic block systems use wayside signals that through position of a flag, arm, or colored light, or different colors of lights, or both, convey information to the train crew about the condition of the track ahead.  These are called automatic because they function without human intervention, whereas a manual block system requires direct human intervention to issue the authority.

A block signal system is a system that combines the features of a block system for train control (as opposed to a time system) and a wayside signal system.  Automatic block signal systems (ABS) were developed in the 1870s but did not achieve real technical maturity until the late 1890s.  Mass installation of ABS systems began in the first decade of the 1900s on U.S. railways.

ABS systems work through track circuits.  In simplest possible terms a D.C. track circuit works as follows.  A current is applied to the rails at one end of a block by a battery, positive D.C. to one rail and negative D.C to the other.  The current travels to the other end of the rail in that block.  At the other end of the block the current travels from the rails via cables to a relay.  The current energizes the relay and holds it in the up (open) position.  When a train enters the block, the current travels through the wheels and axles of the train between the rails and now the current has a shorter and lower-resistance path to travel between the poles of the battery than through the relay.  The amount of current reaching the relay is reduced and is no longer sufficient to hold the relay in the up position.  The relay drops out, closes contacts, and the contacts energize a wayside signal.

Block signal systems look like what you see at trackside today -- colored signal lights on masts at wayside.  Prior systems used physical position of arms (semaphores) and other methods to convey information, but the function and method was identical.

RWM

[chad thomas]

Hi Tony,

The basic concept is that a track is divided into blocks and signaled accordingly to keep trains spaced a safe distance from eachother. If a given block is occupied by a train then as another train approaches that block it will be alerted to that trains presence ahead by the signals. In a simple single track ABS line the approaching train will see (and this varies somewhat) something other then green about 4 blocks from the occupied block. The first would be flashing yellow, the second hard yellow, then reds. ABS (automatic block signals) are a protective overlay and DO NOT convey authority, that is given by the dispatcher with track warrants (TWC) or block authority (DTC) (not the same kind of blocks *see note below). A CTC system also has blocks just like ABS does but in CTC territory the dispatcher can controll the switches and signals at controll points (sideing ends) and the authority IS conveyed by signals. But even in CTC territory  the track is broken down into blocks to keep trains safely spaced.

 

* A DTC block is a section of track between set locations at either end, usually a section of tracks between sideings  (5-20 miles). A signal block is the section of track between signals (1-3 miles).

 

 This might help too:

http://www.alkrug.vcn.com/rrfacts/signals/signals.htm 

 

This is a quick simple explaination so don't beat me up if this is too simple guys. 

[chad thomas]

 (and once again our resident expert gives a much better explaination)

[CShaveRR]

Tony, now that your question has been answered by the late-evening PST shift (I was in bed!), let me ask you a question:

Does your book mention a colission or derailment between two Grand Trunk Western trains in either Nunica or Denison, Michigan, in the first decade of the last century?  (And where does one come across such a book?  I'm a Michigan native, and have never seen it.)

[SW 1200]

Railway Man and Chad Thomas,

Thanks for the help.  Really appreciated. 

CShaveRR,

Yes, the book has a summary concerning the Grand Trunk Western collision at Nunica, Michigan, on January 27, 1911.  The summary also has two photographs of the accident scene. 

The book contains numerous excellent old photos of many of the accidents featured in the book.  The work also contains many diagrams and maps to explain how certain accidents occurred.  The accidents in the book are organized by Railroad Line in alphabetical order, starting with the Ann Arbor Railroad and ending with the Wabash Railroad.  Then the accidents are arranged chronologically within their own specific Railroad Line section.  The authors cite the primary sources and secondary sources of their research. 

Overall, the book is pretty good.  The only thing that I don't like about this book is the lack of an index.  A dedicated enthusiast can easily index it according to his interest or needs (in contrast, the three-volume aviation disaster book that I authored has over 32,000 individual index entries). I highly recommend this book for those interested in this aspect of Michigan Railroad history. 

I purchased the book through the Historic Rail Catalog (800-261-5922)

Michigan Rail Disasters 1900-1940

by Mark Worrall and Benjamin L. Bernhart

Published by

Outer Station Project Publications

1335 Railroad Road

Dauberville, PA 19533

[e-mail]  610-916-2433

Thanks again to all who took the time to help me.

TonyM.

 

 

 

____________________________________________________________________________________ 

Anthony J. Mireles

FATAL ARMY AIR FORCES AVIATION ACCIDENTS

IN THE UNITED STATES, 1941-1945

www.warbirdcrash.com

 

 

[erikem]

chad thomas

 (and once again our resident expert gives a much better explaination)

 

Don't sell yourself too short, you did mention a few things not brought up by RWM. 

[BaltACD]

And the benefits of the best of block signal systems can be eliminated by simple human failure to observe and react to the restrictive singal indications that get displayed.

[CShaveRR]

Thanks for the information, Tony!  I got a catalog from them not too long ago, and found the book listed therein.  It is open hinting season now, right?

[Railway Man]

erikem

chad thomas

 (and once again our resident expert gives a much better explaination)

 

Don't sell yourself too short, you did mention a few things not brought up by RWM. 

And Chad got everything right, too.

RWM

[chad thomas]

Railway Man

erikem

chad thomas

 (and once again our resident expert gives a much better explaination)

 

Don't sell yourself too short, you did mention a few things not brought up by RWM. 

And Chad got everything right, too.

RWM

 

 

You taught me well master  

[nbrodar]

 Not everyone uses the flashing yellow (or double yellow in some books) advance approach.  We had a division officer "famous" for his dislike of advance approach, and refused to allow it's use.  So we either get clear approach stop, or clear approach approach stop, if the blocks are close together.

Nick

[chad thomas]

 Absolutely true Nick, But the answers were simple. An explaination of all the systems / senarials could (and do) fill entire books (but I'm sure I don't have to tell you that). I myself find it all very facinateing.

[AgentKid]

Thank you Railway Man for clearing something up I have wondered about for many years. When I was a kid at Irricana AB my parents were friends with the agent at Morley AB on the CPR mainline Laggan Sub. It had an ABS system before 1967 (approx) installation of CTC. While visiting there I remember seeing the indicator box that would let the agent know that a train had hit either end of the block. A white light would show and an out of tune bell would sound when a train would hit either end of the block. Knowing that ABS was installed that early in the twentieth century explains a lot about the appearance of that box. It always reminded me of an old time wall mounted telephone with the mouthpiece sticking out of the front and the ear piece hanging on one side and the crank on the other. Except that there were two lenses where the two black bells would have been. I never knew ABS technology was that old. Thank you for clearing up one of those confusing memories from my past.

AgentKid

 

[tree68]

Drawing from my limited knowledge of signal systems -  

To expand a bit on "aspects" (how the signals display), semaphores often (always?) came in pairs. One was the "home" signal - what what going on in the block you were about to enter, the other was the "distant" signal - what was going on in the next block beyond (or was it the second block beyond?).  This gives you a minimum of one full block, and possibly two, of clear track since a train about to leave a block would still be showing as in the block. 

As long as the home signals you encountered were clear, you could continue at a pretty good speed, but you had to be aware that the next signal you hit could be red, forcing a stop, or maybe a reduction to restricted speed.

As Chad pointed out, modern systems reach out 3 or 4 blocks.

There are books on signals available.  I haven't looked at Al Krugs signal page (although his air brakes page is phenomenal), but I'm betting it's good information as well.

DC was mentioned as a method of detecting trains in a block.  Later systems used pulsed signals in the rails.  This not only got past the possibility of changing resistance between the rails causing false indications (as sometimes happens at crossings), but because the timing of the pulses could be varied, pickups on the locomotive could detect the pulses and "translate" them into a signal indication for display in the cab - cab signalling!

I'll gladly stand corrected.  We all learn that way.

[nbrodar]

chad thomas

 Absolutely true Nick, But the answers were simple. An explaination of all the systems / senarials could (and do) fill entire books (but I'm sure I don't have to tell you that). I myself find it all very facinateing.

 

You are quite right Chad.  It gets even more interesting with cab signals on the Corridor where you can get a Super Clear.

Nick

[chad thomas]

 A super clear ??? Is that like so clear that you can't even see it (sorry, couldn't resist )

Seriously though Nick, What is a super clear ? 

[nbrodar]

 It's one of the aspects in Amtraks NEC Cab Signals, telling you there are something like 8 blocks ahead clear OK for 150mph.

Nick

[Sunnyland]

Interesting to  read this info.  I can remember  seeing the semaphore arms go up or down when I was a kid and train watching with my Dad who worked for Frisco.  I can also remember riding in a dome car heading to California and seeing the overhead bridge signals change to red as our car passed under it.

[Beyer-Garratt 6040]

I would hate to confuse you, but here in NSW, where we still use semaphore signalling in some places, we can get away with one signal arm doing both jobs. The horizontal position is stop, the same as most railroads; raised to an angle of 45' is proceed with caution, the next signal may be red or at danger; and fully upsight (raised at 90' from the horizontal) is section ahead clear. Each region, country, state or locality has its own method of signalling whether the track is clear or not. We even allow drivers to pass signals at red (after stopping) in some places, and proceed at a slow speed (usually under 15 km/h) so that they can pull up if the road ahead is obstructed.