American safety systems modeling

[rman]

I think the correct way to frame this problem is in terms of blocks, not signal heads.

   Stop              Approach             Approach               Stop
(was Clear)           Medium
    o                   o                    o                    o
    |  [===][===][===]  |                    |                    |  [===][===][===]
----+-------------------+--------------------+--------------------+------------------->
            Clear          Approach Medium          Approach              Clear
                                                        |---------|
                                                        Restricting

Cab signals indicate the current state of the block, which can change instantaneously. Signal heads indicate the status of the upcoming block, and regulate movements between blocks.

This is why cab signals do not show the "next" signal.

[dcarleton]

Okay then, we'll consider this in terms of blocks. The block that you're in cannot be clear because it's occupied - by you!

[ebnertra000]

I would argue that something along these lines is more likely:

http://www.lundsten.dk/us_signaling/apb_ic_cab/index.html

While PRR may not have used the same logic (I have yet to meet a signal system from the NE US that was fully logical), that does seem the most likely setup

[rman]

That's a great find, and it supports what I've been saying! The PRR system wasn't identical, but it was built by the same vendor, US&S.

Notice that the cab signal remains at Clear all the way until the first Approach signal. It does not inform the engineer of the next signal in advance.

[dcarleton]

Sure about that? In the Illinois Central example, in the event that the next block ahead of the train suddenly became shunted (by a malfunction or by human error), the next signal ahead would go to red, the AC pulse in the rails would be interrupted, and the cab signal would go to Restricting. How is that not informing the engineer about the next signal ahead?

[rman]

Well, you got me. We cannot use the route editor to simulate pulse codes and transponders, so we have to account for these conditions in the TCS script.

Nevertheless, the point is that under normal circumstances, an engineer would not expect his cab signal to downgrade until he physically passes the signal displaying the more restrictive aspect. Railworks is correct and MSTS is wrong.

[dcarleton]

Now, starting with the Illinois Central example where there is one AC pulse frequency, extrapolate that out to where there are three pulse frequencies available, as on the PRR. Instead of just one level of immediate protection covering just the one block ahead, now there are three!

[dcarleton]

We cannot use the route editor to simulate pulse codes and transponders

One more thing - You do realize that the PRR system did not use transponders? The system only required a coil in a bar ahead of the lead axle, as shown in the reference material. Very clever, for 1927!

[bobwdude]

Regarding PRR cab signaling - here's a good video on the PRR's safety features (from the PRR) including cab signaling. PRR cab signals showed four aspects and mirrored the current signal aspect, not the aspect of the next block.

[rman]

Good find. I believe this clears up the matter.

[Vince]

Sorry, but every cab ride on the PRR I had (many) the cab signal aspect displayed was the signal ahead.
I don't have ANY faith in the videos produced for general public consumption.
A technical manual would be the only thing I'd want to see. These ad hoc films are not to be believed.

Here's a scenario:
You are running under an approach indication and the next signal ahead is RED. You're saying saying the cab signal aspect is at approach? Sorry but my actual observation is the cab signal in this example shows STOP.

Personal observation and discussions with the engineer are the basis for my position, not some film meant for the general public.
I have more material but it's well hidden in 18 years of collecting for my route building notes and I will search for more material. Meanwhile the signal scripts produced by Signalmaster (Hank Sundermeyer) stand.
Here is a chart from of aspects with cab signals included.
PLaspects56S.jpg

regards,

[Railfan722]

From the 1987 NTSB investigation into the Chase, MD collision:
ntsb report.jpg
Which supports the cab signals repeating the last aspect passed, NOT displaying the next one. The NTSB report also makes note that the signal drops to restricting at a distance of 4,450 ft. from the stop signal. If you want to read the whole report, it's here: https://www.ntsb.gov/investigations/...ts/RAR8801.pdf
The info here is from page 32.

[ragtimer]

Pointless showing the state of the block you are occupying or what was the aspect last passed.On Eurostar/tunnel and presumably on the French side there are no wayside signals,just block indicators either Nf or unmarked Nf means the driver is not allowed to pass at danger unless authorised.Cab indication is for 7 blocks ahead.Search for "tgv cab signalling"

[bandorr2000]

Absolutely correct Vince, the cab signals ALWAYS displayed the block ahead.

[dcarleton]

Good find. I believe this clears up the matter.

Sure about that? Intriguing video, but as literature more of a coloring book than a text book. Still, if taken as documentary, pay special attention to exactly what the narrator said (at 3:59 to 4:11): "These are the cab signals. They reproduce the wayside signals. In fair weather or foul, in sleet or fog our engineer knows at all times the conditions ahead."

Notice that the narrator specifically said conditions ahead. Looks like Vince may be on to something!