cab signals

[Mr Downtown]

I'm trying to better understand CTA's cab signal system.  My understanding from Bill Vandervoort's wonderful web page about it is that the cab display shows four aspects: green, yellow, red, and flashing red, indicating the condition of the block the train is just about to enter. But traditional wayside signals, as I understand it, showed not only the condition of the block about to be entered but also the block beyond that so that trains could slow before entering an occupied block.

I guess my question is whether this is a significant difference.  As I understand it, CTA trains get a yellow-35mph signal when the block beyond is occupied, and this is presumably done by the occupied block sending its condition back a block so the circuitry would show a yellow rather than a green.  But that same logic could have been used for wayside signals, where instead it was thought necessary to show two blocks ahead.  Was this simply a judgment call by CTA in the 1960s that their braking was sufficient to not need the same advance warning that steam roads traditionally used?

[Busjack]

chicago-l.,org appears to have a more complete explanation.

Probably the main distinction is that the ATC will stop the train, while on the mainline railroads, it will take PTC to do it. Also, that page points out that ATC is continuous instead of being based on blocks, so the CTA system must be based on being so many feet behind the other train.

 

[Mr Downtown]

Well, that also puzzles me. I'm pretty sure CTA uses fixed blocks, not the moving blocks possible under more modern systems, so it's not based on "so many feet behind the other train."

In addition, Bill writes "CTA uses a high frequency cab signal system which functions without insulated rail joints separating the blocks."  How exactly would that work?  How would a train only pick up the signal from the block it's about to enter, and not from the next one ahead?

[Busjack]

8 hours ago, Mr Downtown said:

n addition, Bill writes "CTA uses a high frequency cab signal system which functions without insulated rail joints separating the blocks."  How exactly would that work?  How would a train only pick up the signal from the block it's about to enter, and not from the next one ahead?

If there are any blocks. they are defined by the different frequencies. What I got from his page is that the train wheels short out the radio signals, and if the follower gets no frequency, the red light comes on.

[Mr Downtown]

Huh?  The same frequencies are used systemwide.  And if there are no insulated rail joints, how does the train keep from shorting out the very signal it is receiving?

[Busjack]

8 minutes ago, Mr Downtown said:

Huh?  The same frequencies are used systemwide.  And if there are no insulated rail joints, how does the train keep from shorting out the very signal it is receiving?

I don't know if you want to find out or not, but if the source you cited isn't adequate for your needs, based on your preconceptions, I suggest you contact Alstom Signal in West Henrietta, NY.*

______

* Under Rochester.

[andrethebusman]

Back when CTA used wayside signals in the subway, the distances betwern signals varied widely. Between stations far apart, in stations close together. How many reds were behind a train varied by location, with two usually between stations, as many as four within a station. Remember all except the last one would clear (drop the trip arm) when following train stopped in front of it, permitting follower to come up close to leader. Normally this was only done in stations, but it could be done anywhere if needed, such as to come up to a disabled train. 

ATC works the same way, except a red clears to a flashing red after stopping. 

[andrethebusman]

CTA does have defined blocks. Those yellow number signs like RV1 145 between tracks are block limits.

[Mr Downtown]

Setting aside the snark and complete guesswork offered in this thread, I talked with some experts and I think I understand better now. 

I was having trouble understanding how the blocks could be isolated from each other without some kind of electrical insulation. While it's technically accurate to say there aren't insulated rail joints between blocks, there are instead "impedance bonds" that prevent the audio frequency signals from traveling between blocks but do pass the 600VDC traction power.  Isolating high-voltage running rails requires big hefty nonconducting blocks while low-power AC signals—not unlike what a big stereo amplifier sends to the speakers—can be blocked more easily.

So at each block limit, the logic board looks at whether a train is occupying the block(s) ahead and injects the proper signal: yellow-35 mph, yellow-25 mph, or yellow-15 mph.  If a train enters an occupied block, where its leader is shunting (shorting out) the injected signal, no signal will be received and the cab signal will "fail" to red.