You might recall at our last meeting that I said something somewhat harsh about our British cousins' tendency to keep Wheel Protection Systems active during emergency braking.
Maybe you don't. I don't hold it against you, being at an age when I forget what I just remembered what I forgot a little while ago.
Anyway, I subscribe to our UK colleagues' government's Rail Accident Investigation Branch (RAIB) website which updates me on the latest incidents under investigation, and completed investigations.
My understanding is that the RAIB functions akin to our NTSB, if the NTSB were exclusively devoted to rail, and the NTSB could reach a conclusion in less than a year.
Anyway, just 13 days after I related the tale of woe regarding the collision on the Norristown High Speed Line of SEPTA, a notice showed up in my inbox announcing that RAIB had issued its interim report, including initial findings of key factors contributing to the accident:
Below is a description of events and actions leading to the accident:
21. At 18:41:09 hrs, 1 minute and 47 seconds before the accident (timings in this section are taken from the train’s OTDR, adjusted to synchronise with the time recorded by the signalling system), the driver of train 1L53 shut off power and allowed the train to coast down the prevailing 1 in 169 gradient. Ten seconds later he acknowledged the AWS warning for signal SY29R, which was showing a double yellow preliminary caution aspect. At 18:42:03 hrs, around 1,600 metres after passing signal SY29R and with the train travelling at 86 mph (138 km/h) on level track, the driver made a step 2 brake application. This was in accordance with his usual practice and was done with the intention of being able to stop the train at signal SY31, which was at this point around 1,500 metres away. Although the train’s speed began to reduce, analysis of OTDR data shows that its wheels began to slide almost immediately after this brake application was made. The driver made a full-service brake application five seconds later and moved the brake controller to the emergency brake position after a further six seconds.The train’s wheel slip/slide prevention (WSP) system was active throughout this braking, but the train’s speed reduced only slowly.
22. As the train approached signal SY31, which remained at danger, the Train Protection and Warning System (TPWS) fitted to the train detected that it was travelling above the set speed of the overspeed sensor (OSS) system fitted on approach to the signal, 34.5 mph (55.5 km/h). The TPWS system therefore made an emergency brake demand. This had no effect on the degree of braking demanded, because the maximum available braking had already been applied by the driver.
23. Train 1F27 passed across and clear of Salisbury Tunnel Junction less than 40 seconds before train 1L53 arrived. Subsequently, the driver of train 1L53, which was still sliding and rapidly approaching signal SY31, saw train 1F30 appear from the left and move into the path of his train. Train 1F30 was at this point travelling at 20 mph (32 km/h).
24. At the point where the two trains collided, train 1L53 was travelling at between about 52 and 56 mph (84 and 90 km/h). Train 1L53 struck 1F30 on its right-handside near the front of the fourth coach, as both trains crossed the junction and as1F30 was entering Fisherton tunnel.
The initial findings of the investigators include this:
36. The railhead was found to be contaminated throughout the areas surveyed with a black deposit. This deposit consisted of leaf material which had been crushed under the wheels of passing trains and which is often associated with low adhesion conditions. Analysis showed that many areas had a medium or heavy level of contamination, and that the thickness of the leaf deposit was relatively consistent through each of the areas surveyed. It is likely that the rails were wet at the time of the accident. Wet values of the coefficient of friction measured as part of the post-accident survey were found to be between 0.2 and 0.02, suggesting that there was low friction between the wheels and the railhead. The average rate of deceleration of train 1L53 suggests adhesion levels closer to the 0.02 value were prevalent at the time of the accident. Samples taken from the area around signal SY29 and between there and the junction showed that the deposits were smeared and flaky and typical of the sort of contamination layers which exist after train wheels have slid over them.
Got that? Train 1L53, approaching a stop signal ("at danger" in the UK) applied the brakes with a train speed at 86 mph and slid approximately 1691 meters (estimate 1500 meters to signal SY31, additional 191 meters to point of collision) despite an operated initiated emergency brake request, with the wheel slide protection system through all braking requests. Examination of the railhead showed an accumulation of crushed leaf matter, possibly reducing adhesion between wheel and rail by 95 percent.
Railroading uses simple math which is a plus for me since I use simple math all the time. So picking up pencil and pen and my trusted New York Yankees Sharp EL-525 calculator, I figured the following -- converting the metrics to our typical scale. All errors are mine although I'll deny it if pressed.
The distant signal governing approach to the interlocking is set approximately 10171 feet to the rear of the signal displaying "stop."
At the maximum authorized speed of 90 mph, the required rate of deceleration is .86 feet per second per second.
The type of Train Protection and Warning System in operation in this incident apparently has an unrestricted "free run time," not requiring or enforcing a certain rate of deceleration or applied braking effort within X seconds of the warning of necessary speed reduction. In the US, this restriction on free run time and requirement for a mandatory deceleration is what distinguishes speed or train control systems from simpe cab signal systems. The free run time, known as "delay time" is limited by regulation to no more than eight (8) seconds.
The train operator does not initiate any braking effort until approximately 4921 feet to the rear of the stop signal with the train speed at 86 mph. The required deceleration rate of the train to avoid a signal overrun is 1.61 feet/sec/sec, or about 1mph/sec, well within the range of normal, even "light," braking.
However with the low adhesion between wheel and rail and wtih the WPS active throughout, the train travel 5548 feet, decelerating to approximately 54 mph before colliding with train 1F30. The actual achieved rate of deceleration to the collision point is approximately .086 feet/sec/sec, one-tenth of the signal design requirement.
Certain things are painfully clear to the most casual observer. In order of importance:
(1) low adhesion made this collision possible
(1) keeping the WPS active when emergency braking is requested or initiated made this collision actual
(1) A single signal block of extended length governing the transition from 90 mph to 0 velocity is a design that practically defeats the purpose of signal blocks, which is to regulate train speed in order to maintain safe train separation. Indeed a single signal block of extreme length that encompasses the transition from any unenforced maximum authorized speed to 0 velocity compromises the safe separation of trains.
(1) The accident demonstrates (1) above. The fixed location for a penalty application when the train speed exceeded 34.5 mph was incapable of preventing the collision because the WPS was active and the speed of the train at the penalty point would overrun that allotted by design.
(1) Intermediate signals requiring and enforcing graduated speed reductions, such that train speed is never allowed to exceed that authorized by the signal indication enhances both throughput and train safety. Think about it.
(1) Just to be clear, WPS should never be active when emergency braking is initiated. Flatten the wheels, not the passengers.
David Schanoes
February 27, 2022
Name That Tune
Willie and Millie Got Married Last Fall
They had a little Willie Junior and that aint all
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