Help me understand railway signals

[LookingGlassTie]

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Got a question about signal blocks:

As I've mentioned before in other threads, I took a trip from Newport News, VA to Baltimore, MD in June 2017. I took Northeast Regional 94 from NPN to BAL on June 15 and took Northeast Regional 83 from BAL back to NPN on June 16.

It was on June 16 that a trespasser (I believe) was struck by the 90 Palmetto around Lorton. Our train was held by CSX dispatch at ALX.

So the question is: Was our train in the same signal block as the Palmetto or were the two trains in different (perhaps even adjoining) blocks? Would that explain why our train was held at ALX as opposed to being allowed to proceed a little farther south?

(I should add that as a result of the delay, I didn't actually arrive back in Newport News until really early on June 17. )

Thanks!

 

[Shortline]

Unless there is a a specific Dispatching rule that prohibits allowing a passenger train into an occupied block, the answer is, could be either one. When there is an incident, and trains are stacking up until it's cleared, it's not uncommon for the dispatcher to "talk a train by" an absolute stop signal. That train would then be following the preceding train at Restricted Speed, and simply pull up and stop behind it. Or, if the preceeding signal was an intermediate, the occupied block would just be entered by either a restricting signal, or a stop and proceed red signal, and again, simply enter the block, looking out for the train ahead.

Or the preceding train was in a separate adjacent block. Just not enough info to know. But it's certainly possible, either scenario could be correct.

 

[jis]

They may also just have preferred to hold a passenger train at a station instead of in the middle of the sticks, where it would be hard to get to should a passenger emergency of some sort arise.

 

[Thirdrail7]

I just want to make sure I'm following this.

. I took Northeast Regional 94 from NPN to BAL on June 15 and took Northeast Regional 83 from BAL back to NPN on June 16.

On June 16th, you were heading south on train 83, towards NPN.

It was on June 16 that a trespasser (I believe) was struck by the 90 Palmetto around Lorton. Our train was held by CSX dispatch at ALX.

Northbound train 90 struck something around Lorton and your train was held in ALX.

Two things come to mind. One is they likely held you in a station for accessibility reasons. The other thing is you have a northbound train and a southbound train.

I would sincerely hope they would not have had two trains heading in opposite directions in the same block.

 

[MARC Rider]

I would sincerely hope they would not have had two trains heading in opposite directions in the same block.

Well, it might be possible if the dispatcher was Uncle Fester.

 

[Jovet]

Unless there is a a specific Dispatching rule that prohibits allowing a passenger train into an occupied block, the answer is, could be either one.

There is. Sorta. An absolute block is a state where the dispatcher guarantees a train sole access to its signal block. These are common for passenger trains to reduce risk of human error and collision. But these have to be agreed upon, and often requested. But it's likely that trains were simply held to keep the scene clear.

I think the OP probably got his signaling questions answered. But, North American signals can be rather confusing without any grasp of their core logic. Best explanation that I urge everyone to read is this: http://web.archive.org/web/20160313144451/http://alkrug.vcn.com/rrfacts/signals/signals.htm

 

[Jovet]

Actually, just in case, I'll answer these in my own way. Even though they were asked 8 years ago.

1. In the SDU Display column, what is meant by "A green band"?

It's specific to the type of speed display you'll find in one of their locomotives.

2. For the restricting signal and stop signal, how do I know whether to go 20mph or completely stop?

You proceed until you have to stop. As others have already explained, Restricted Speed is not a fixed speed limit, but changes moment to moment based on track conditions. It puts all the onus on the driver (engineer) to safely control the train and stop before any danger or inconvenience to the train.

1. What does any of this mean? If the fixed signal indicates cab speed, my cab signal will be either "clear", "cab speed", "approach limited", or "approach medium"? How can it be more than one?

The fixed signal is the permanently-installed signal encountered along the track the train passes. A fixed signal can be a sign or a yellow light or whatever else is depicted in the Rulebook.
If the fixed signal shows Cab Speed, then it means the train should be governed according to what its onboard signal or speed display shows. It also means if you encounter a Cab Speed signal, and the cab signal display does not show any of those cab signal aspects, then something is wrong and you can't trust it.

2. What does it mean to just "approach"? What speed so I approach? I understand that approach medium is approaching the next signal at half of normal speed, but what the heck does approach mean?

"Approach medium" does NOT mean go half of normal speed. Such signaling is of the speed signaling paradigm. This means that signal aspects directing trains to change tracks will always require a specific speed limit, based on their signal aspect. There are three standardized speeds that such signals can indicate: Slow, Medium, and Limited. Approach Medium means you must not exceed Medium speed by the time and while passing the next signal. The actual speeds will vary by railroad and region, but generally Slow is 15 MPH, Medium is 30 MPH, and Limited is 45 MPH. As a verb, "approach" means "to continue and arrive upon". You arrive upon the next signal, which this signal is warning you about. As a signal rule, Approach (by itself) means the same as "Approach Stop". Some rulebooks specify a speed you must slow to upon accepting (passing) an Approach signal (e.g. "reduce speed to not exceed Medium speed"); others do not. Similarly, an Approach Medium rule usually will not state how fast you need to slow down to immediately, it only states you must not exceed Medium speed at the next signal. It is up to the engineer's knowledge of the territory and train to comply with this rule. Conversely, a Medium Approach (means the same as "Medium Approach Stop") says how fast you can only go now (Medium speed) and how fast at the next signal (Stop).

Hope that helps someone.

 

[Shortline]

There is. Sorta. An absolute block is a state where the dispatcher guarantees a train sole access to its signal block. These are common for passenger trains to reduce risk of human error and collision. But these have to be agreed upon, and often requested. But it's likely that trains were simply held to keep the scene clear.

I think the OP probably got his signaling questions answered. But, North American signals can be rather confusing without any grasp of their core logic. Best explanation that I urge everyone to read is this: http://web.archive.org/web/20160313144451/http://alkrug.vcn.com/rrfacts/signals/signals.htm

Right-I get that. I understand signals quite well. But it’s not uncommon to put two trains or more, into an absolute block when traffic backs up for some reason. The first enters in signal indication and stops at the next absolute, and following trains can be talked by the absolute stop signal and essentially pull up behind the stopped train ahead. I was just not sure if there was a rule that prevented dispatchers from doing that with a passenger train. It’s not uncommon for a freight train.

 

[Jovet]

Right-I get that. I understand signals quite well.

I apologize for the confusion. An absolute block has nothing to do with signals (absolute or otherwise), nor anything to do with absolute block signaling (common in Commonwealth countries).

 

[saxman]

This is the best video I've seen about signals and how to read them on CSX. He has about 6 of these videos about signals plus other railfan videos with excellent narration and drone footage.

 

[Jovet]

This is the best video I've seen about signals and how to read them on CSX. He has about 6 of these videos about signals plus other railfan videos with excellent narration and drone footage.

I agree Mr. Harmon does a stellar job of railfanning videos. The best on the internet I've ever seen! His signal videos featured here do show off many examples of the various aspects, but tend to gloss over whys and whens. The site I linked above explains the logic, so combinations of colors don't have to be just learned by rote. There are also a few incompletes/mistakes, which I tried to address in the comments.