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inflammable, on 10 November 2012 - 03:49 PM, said:
If they're permissive signals, then they'll return to green when the second block is clear.
Alas, that's not always true.
inflammable, on 10 November 2012 - 06:54 PM, said:
For a Permissive, the red signal indicates Restricting.
That is technically wrong. But it depends on the railroad. See below.
inflammable, on 10 November 2012 - 06:54 PM, said:
If there's a broken rail, then you may be able to spot it before you roll over it. If not, you're still going slow enough to where you will probably make it all the way over
When operating under Restricted speed you're expected to stop before anything that is a danger to your train. If you can't stop in time then you're exceeding Restricted speed.
nyc01, on 15 November 2012 - 12:56 AM, said:
I think you first have to understand that a dispatcher has no direct control over an automatic signal in CTC, it's only the control point/interlocking signals that the dispatcher has direct control over.
That is correct, but the idea applies to any traffic control system, not just CTC.
wacampbell, on 10 November 2012 - 06:08 PM, said:
I am confused about these permissive signals ( even after reading the wikipedia explanation ). Under what conditions does it make sense to allow the train to proceed past a red signal? Doesn't the red signal mean there is something blocking the track, or even an oncoming train? It seems dangerous to let the trains just ignore the signals and carry on. Why have them at all, if the train just ignores them anyway?
It will be helpful to first understand that the purpose of an absolute signal is to avoid accidents, property damage, and loss of life. An absolute signal is used where train paths intersect or cross. Trains are not automatically permitted to proceed past an absolute signal at Stop because it'd be just like pulling into a traffic intersection when you have a red light. If you must stop for a Stop signal then it's likely another train has a cleared path that conflicts with yours. You can only proceed if a dispatcher or control operator formally tells you it's okay to do so.
The only real difference between an absolute signal and a permissive signal is that a permissive signal does not usually show Stop. Instead, when it is red, it indicates a "lighter" indication like Stop and Proceed or Restricting or Restricted Proceed. The actual meaning depends on the specific rules of the railroad. As James said, a train can't ignore a permissive signal showing Red. But the situation with a permissive signal at Red does have a different connotation. While a collision and loss of property or life could still occur, it isn't automatically going to happen. If the railroad has a Stop and Proceed rule, then the train must stop before passing the signal, just like an absolute. The train may have to wait a certain number of minutes. Then it can proceed at Restricted speed. If the railroad has a Restricting or Restricted Proceed rule, the train does not need to stop but must not pass the signal exceeding Restricted speed. Remember that Restricted speed isn't a fixed speed limit, but a floating one which can change moment by moment (with an upper limit that varies by railroad). In Europe this is often called Driving On Sight. A train operating at Restricted speed MUST be able to stop within half the range of vision ahead of it. That means if the engineer can see 1000 feet down the track then at that moment he must be going slow enough to be able to stop within 500 feet. Any problem can cause the train to need to stop, including a red flag, a lighted fusee, a broken rail, a Stop signal, a "stop" hand signal, a turnout not lined to normal, or obviously another train or cars on the track. Restricted speed puts all the burden on the train crew to ensure the safety of the train as they proceed. This is all akin to approaching a Flashing Red traffic signal in the United States: you must stop, then may proceed if it's safe to do so--the burden is on you.
The point of this is to allow trains to keep moving (albeit slowly) when not faced with an absolute signal. If the train has to stop, then it has to stop. But if it can keep moving then it will get to its destination faster, save fuel, etc. This is especially useful for trains following one another in the same direction on the same track.
The terms "permissive" and "absolute" signals area universal concept. No one type of traffic control system, such as CTC or ABS, "owns" these terms. CTC, by its nature, has absolute signals, and usually has permissive signals. ABS, APB, and DTC, by their nature, have permissive signals, but may have some absolute signals (such as siding exit signals), too.
CTC is the only traffic control system that provides for warrant authority via signal. If you are operating in Absolute Block Signaling (ABS) rules, the block signals are there for added safety, and you must obey them, but they don't let you occupy a specific track. The dispatcher must still formally communicate to you the limitations of your authority. With CTC, however, a non-Red (absolute) signal is a non-verbal warrant of authority to occupy that track. You can continue to freely operate on that authority until you encounter an absolute Stop signal. This is immensely efficient for trains and traffic operations, as you can imagine. It's also why CTC is so expensive.
As mentioned above, in CTC, the dispatcher has direct control over the absolute signals which are said to be at control points. Most control points form an interlocking. An interlocking is simply a location where train paths cross which ensure safety of trains. Each interlocking can be unique, but the idea is that the dispatcher arranges "paths" for trains through the interlocking. An interlocking can be as simple as one track going to two for a siding, or as complicated as 30 tracks meeting and crossing over one another in all sorts of directions. If the dispatcher were to try to clear two paths at the same time that conflict, the system would reject the dispatcher's second request. If the dispatcher requests that a switch change position, and it jams half way (becomes "out of correspondence") then the signal will never clear because the cleared signal signifies that the path is set, safe, and locked all the way through the interlocking. The dispatcher does not really have direct control over the signals, but merely requests the switches and signals to clear as needed. An absolute signal that is "cleared" is not displaying Stop (British parlance: the signal is off). The dispatcher also can't directly control--or even see--which aspect actually shows on the signal as that depends on the track conditions. Most control point signals are also block signals for the next signal block. Thus, if the following signal is showing Stop and Proceed, then the control point signal would clear to Approach and not Clear.
When a train accepts a cleared absolute signal (by passing the absolute signal), it "owns" the path through the interlocking as long as it is within it. The dispatcher can no longer change or revoke the path. The signal that allowed the train to enter the interlocking is restored to Stop. Once the train leaves the interlocking, the path is cancelled. With no paths set and locked, all signals at the interlocking show Stop. Some CTC systems allow the dispatcher to choose to not cancel a path once a train has used it. This is called fleeting. Say you have 5 trains moving in the same direction and you know you want them on the same tracks. If you are the dispatcher and you don't want to have to setup the paths through all the interlockings each time, you could tell the CTC machine to "fleet" the path through an interlocking so that it remains active until you cancel it. In this case, the absolute signal changes to Stop when it's accepted, but then upgrades as track conditions permit. So, once the first train has moved into the second signal block, the interlocking signal will upgrade to Approach, etc. Modern computerized CTC systems allow dispatchers to plan and setup all sorts of paths long before they're actually needed. The paths are still only executed if field conditions determine it's safe to do so, however.
Between control points can usually be found permissive signals, specifically called intermediate signals within CTC territory. These signals are totally automatic and operate solely on track conditions. The behavior of permissive signals once trains pass them varies greatly. Most permissive signals are setup to be sensitive to a "direction of traffic". Signals for traffic that oppose that direction of traffic show their most restrictive aspect (i.e. Red) as long as that direction of traffic is en force. Some permissive signals may be released from that direction of traffic as trains finish passing them, others will not. It really depends on the philosophy of the railroad, the era in which the signaling was engineered, the type of traffic control system, the specific track/control point arrangement(s), etc. In CTC territory, which is always bi-directional on each track, the direction of traffic of a track is usually set automatically by the appearance of a train in the signal block past a control point.
Finally, remember that in the US permissive signals have number plates while absolute signals do not. That is generally the only visual distinction, though historically permissive signals with more than one signal head were setup to offset the signal heads to make them distinctive at a distance. This concept has unfortunately faded away over the years. But, in Canada, all signals actually have number plates, and permissive signals are indicated only by that offset of signal heads (or if only one signal head, absolute signals have an "A" sign). The one situation where a dispatcher really has direct control over a signal is when a signal without an interlocking is made absolute. Such signals are often former-intermediate (permissive) signals which have been changed to absolute signals forming a control point. Such signals are usually called holding signals. With no interlocking, the dispatcher basically can control at a whim whether the signal shows Stop or something better. Such signals allow the dispatcher to better control trains' approach to often-congested areas, which can help spread-out a bottleneck. Look closely at those "intermediate" signals, because if they don't have number plates, then they're holding signals!
wacampbell, on 10 November 2012 - 08:03 PM, said:
Is MSTS able to properly represent these permissive signals?
Yes. Though, AI trains don't understand Restricted speed, and historically have been stuck at the reduced speed limit set in the signal configuration file.
