Q:When Is Positive Train Control Not All That Positive?
A: When We Move From The Law To The Regulation.
The Rail Safety Improvement Act (RSIA) of 2008 mandates the application of Positive Train Control systems to large portions of the US rail network. The law also authorizes the development of regulations to enforce this mandate:
REGULATIONS.—The Secretary shall prescribe regulations or issue orders necessary to implement this section, including regulations specifying in appropriate technical detail the essential functionalities of positive train control systems, and the means by which those systems will be qualified.
The law also defines exactly what the Congress means when it says PTC:
POSITIVE TRAIN CONTROL SYSTEM.—The term ‘positive train control system’ means a system designed to prevent train-to-train collisions, over-speed derailments, incursions into established work zone limits, and the movement of a train through a switch left in the wrong position.’’
The operative words here are "positive" and "prevent."
Positive has at least 14 definitions according to the Oxford English Dictionary, but the one that concerns us is the railroad definition. In the language of railroads, positive means "absolute," without qualification as in "positive (absolute) stop, which definition happens to correspond very nicely with the second definition provided by the OED:
Explicitly laid down; admitting no question; explicit, express, definite, precise; emphatic.
And "prevent"? Another dozen or so definitions, but the ones that concern us are nos. 7-12, all having to do with stopping something before it occurs:
7. Forestall or thwart by previous or precautionary measures
8. Preclude from or deprive of a purpose
9. Provide beforehand against the occurrence of (something); make impracticable or impossible by anticipatory action; stop from happening
10. Frustrate, defeat, make void
11. Stop (something) from happening to oneself...
12. Cause to be unable to do or be something, stop..
Positive, prevent. Explicitly, emphatically, without question stop from happening.
Sometimes it's important to have a dictionary close at hand, just in case we forget what words are really supposed to mean.
In its final regulation regarding PTC, subpart I, 236.1001-1049, FRA strays a bit from these definitions, for reasons which I'm sure are very valid in the minds of those writing the regulation; to those responding to the input from railroads, suppliers, consultants etc. etc.
Regarding train-to-train collision, FRA specifically accepts something less than positive prevention of train-to-train collision:
(f) Train-to-train collision. A PTC system shall be considered to be configured to prevent train-to-train collisions within the meaning of paragraph (a) of this section if trains are required to be operated at restricted speed and if the onboard PTC equipment enforces the upper limits of the railroad's restricted speed rule (15-20 miles per hour.)...
In place of positive prevention, we have restricting the speed at which the collision can and will occur.
Does enforcing the "upper limit" of "restricted speed" prevent collisions? It hasn't so far, at least not in the 70 or so years that cab signal/speed control systems have been around.
Enforcing that "upper limit" certainly reduces the likelihood of collision, and will limit the damage if a collision occurs, but the so-called upper limit of restricted speed is the non-vital portion of the railroad's restricted speed rule.
The vital portion, the basis for authorizing movement at restricted speed is the non-numerical qualification of restricted speed... "prepared to stop within half the range of vision, looking out for....." and unless we're going to do something very complicated with machine vision, there's no way PTC can enforce that.
But because restricted speed exists; because railroads have always used restricted speed as a measure of crew responsibility; because restricted speed can be displayed as a signal; because restricted speed is used to authorize everything from movement over a #6 turnout to following another train in a block, this restricted speed, this permissive condition has been declared functionally equivalent to positive prevention.
Stop me anytime if I'm wrong. Just make sure you're positive.
FRA repeats this process when transferring the law into the part of the regulation governing prevention of trains operating through improperly lined switches.
Here's how positive prevention gets translated by the regulation:
When a main line switch is unknown or improperly aligned for the train's route in advance of the train's movement, the PTC will provide warning of the condtion associated with the following enforcement:
(1) A PTC system shall enforce restricted speed over any switch:
(i) Where train movements are made with the benefit of the indications of a wayside or cab signal system.....; and
(ii) Where wayside or cab signal system or other similar appliance, method, device, or system of equivalent safety, requires the train to be operated at restricted speed.
For those who think that "restricted speed" enforces train separation, the above sounds eminently reasonable.
For those who know that restricted speed does not enforce train separation and cannot itself be enforced, the above sounds like a big hole in the fence of positive prevention, a hole large enough to drive a locomotive through.
Now the railroad's have interpreted FRA's regulation to mean that, in wayside or cab signal territory, the PTC need only monitor the signal that "protects" the switch and not the position of the switch itself.
For example PTC-220 LLC, the joint venture of the UP and the NS that "owns" the 220 Mhz spectrum dedicated to PTC, has stated in its presentation to the FCC:
Monitoring of intermediate signals also provides a method to meet the requirement to prevent unsafe train movement through switches in improper position. The FRA recognizing that switchers are already interlocked with a signal system in existing installations, specifically recognizes this method in its regulation. Monitoring of intermediate signals elminates the need to directly monitor each and every switch in signaled territory.
Not really. Technically speaking not all switches are actually interlocked with signal systems in existing installations. Again it's a matter of the real meaning of words, and interlocking and interlocked have specific meanings in railroad operations. More on that later.
Not really. The switches "protected" by intermediate signals are not actually interlocked with anything, unless of course the switches are connected to specific systems that provide interlocking.
More ofthen that not, the switches are only connected to the track circuit, that current and voltage flowing through a section of track that when interrupted, "shorted" by the presence of a train or some other condition, transmits that interruption as occupancy of the block.
In turn, that interruption, that condition of occupancy cause the signal to change its aspect, conveying this interruption as an indication to an approaching train to reduce its speed.
The switch is not, by itself, interlocked with the signal, as nothing about the condition of the signal automatically prevents the operation of the switch.
Neither can any condition of the track circuit prevent operation of the switch, unless that switch is equipped with mechanisms that only allow its operation under certain conditions.
Let's look at PTC-220's scenario again. The PTC system only monitors the signal, not the position of the switches themselves. The signal only registers the opening of the switch as an electrical interruption, and displays the exact same aspect is displays when the block is already occupied by a train.
So what happens if the switch is opened after our train enters the block? After our train passes the signal?
The signal already registers occupancy; the signal indication has already changed to "restricted speed" because our train occupies the block.
What happens? Nothing happens to the signal. What about the PTC system? Since it is not monitoring the position of the switch, but only the signal, and the signal cannot register the opening of the switch, another big fat nothing happens. PTC has nothing to enforce.
What happens to the train? If the train is fortunate enough to be operating in cab signal/automatic speed control territory, the cab signal will drop to restricted, and the locomotive driver will have to apply the brakes at a sufficient effort to forestall a penalty brake application.
Whether or not the locomotive operator or the penalty brake application can stop the train before operating over the improperly lined switch is a matter of..... chance.
Does that sound like positive prevention?
And here's the irony, for those big on irony, if the Class 1 railroads had cab signal/automatic speed conrol systems installed over their lines, the Congress would never have mandated, would never have been compelled to mandate, PTC.
So without the cab signal/speed control system, our train traverses the improperly lined switch possibly into the rear end of another train, or into a building.
To be fair, and those of us big on irony are usually big on being fair, FRA has included as part of the regulation that the PTC system must enforce a positive stop short of an improperly lined switch if:
the allowable speed is over 20 mph
movement is made without the benefit of the indications of a wayside or cab signal system
movement would create an unacceptable risk. Unacceptable risk includes conditions when traversing the switch even at low speeds, could result in direct conflict with the movementof another train...
I don't know of any situation where traversing an improperly line switch does not entail the risk of direct conflict with the movement of another train.
I don't know of any situation where movement over an improperly lined switch poses an acceptable risk.
When train and engine crews operate their equipment through improperly lined switches, railroads assess discipline for that violation, as the railroads should.
An improperly lined switch, by definition, presents the possibility of conflicting movement with another train, as the improperly lined switch means a train can occupy, overlap, another train's authority for movement.
That's the reason railroads design and install interlockings which prevent such conflicting movement. That's why the interlockings are designed and defined to/as:
An arrangement (or interconnection) of signal and signal appliances such that their movement (operation) must succeed each other in a predetermined (proper) sequence, assuring that signals cannot be display simultaneously for conflicting routes, and for which interlocking rules are in effect.
What FRA has done with its regulation, to make the regulation "functional," "operable" is to attach the limitations and inadequacies, of the existing train control systems to PTC systems.
Positive prevention has been overriden by permissive conditions.
Next, we'll explore the root cause of the inadequacies of the present train control systems and how PTC is capable of overcoming those weaknesses .
April 4, 2012
End Part 1
Recommendation from a satisfied customer:
"Dave's worth his weight in gold at the pre-1971 fixed rate of convertibility."
Reference available upon request.