ATO and Weather

I was on an EB Epping bound 92 stock the other day and it was totally pouring down with rain.

I got on at Mile End and was going to Leytonstone, between Stratford and Leyton just after exiting the portal the 92 started to rapidly jumping all over the place, the train was in ATO and the driver quickly switched modes to Coded Manual operation because of the jumping. (I could by the way the train was being driven, also the ATP chimes were going off)

Technically are drivers allowed to switch modes whilst the train is in motion? I know for certain CM - ATO cannot be done whilst in motion.
Also the TBC would be stowed if the train was to change from ATO - CM, until the T/Op pulls it out and twists it he would get an Emergency Application. (which on my service did, it was Jumping - (change to CM) - Emergency Application - Motoring, but it was only momentarily)

Or do something to drop the start hold circuit out such as opening the M door. But yes, it's only possble to change modes while at a stand.



I wouldn't be too sure, having worked on the issue of Platform Overruns and Technical SPADs, the overriding opinion is that driving in CM normally results in a less agressive driving mode with the brakes being applied more gently earlier in the inter-station run, an option that ATO doesn't have.

Not doubting that at all, but what I was trying to get at is if a site is a known risk of wheelslip and overruns (like the Met in Autumn), a driver would tend to be more gentle in their braking so as to minimise the risk of locking the wheels.

To get this back to the original point of driving the train whilst the track conditions are wet, IMO (and the evidence from CM runs has backed it up) you're less likely to overrun the PAC because you would brake earlier and more gently in anticipation of the problem, rather than braking as you would with good rail conditions and running the risk of the wheels locking.

so far on this thread nobody seems to have answered the original question, which intrigues me as well.

If the rails are wet, as stated above a driver will start braking earlier and more gently. So how does ATO know to do this? Or does it do what some ABS on the mainline used to do and gently drift past the stopping point, hitting the buffers if necessary Can someone explain this, because there is obviously (presumably) something in the control system to prevent it?

Upminster doesn't have them either (as the mainline carries straight on into the depot) except for platform 5 which had a 10mph trainstop put in quite recently - something to do with protecting the works to put in a connection with Network Rail for D stock refurb transfer, I believe.

To further elaboarte on things a bit, some ATO-operated systems around the world can and do switch between various operating modes, governing the general characteristics of the train's acceleration and retardation, among other factors. Sometimes, these modes are intended for various levels of passenger loading and, in other places, such as Singapore, they are used in cases of heavy rainfall or monsoon.

However, the trains still do not recognise and plan ahead -they merely operate within different parameters. Which is where a well-trained human driver can truly excel.


/Igelkotten

At the end of both the platforms you have got shunt signal to the yard, which would stop you if you came in a bit too fast.

Goung back to the original question, ATO doesnt know what the rail conditions are, there for it will not take into the account that it may lock up because it may be wet. However if it does lock up, there are systems on the train to allow it to stop in the correct place. If it knows the wheels have locked up and there is no way of getting out of it, it will dump the emergency brakes on, an example of this happens quite often at East Acton EB.

Well, you'd certainly stop before you ran out of road!

Ah yes. Perhaps I should revise my statement to read 'dead end terminal stations'.

E&C, Walthamstow and Brixton are funnies again, the speed control there is to protect the Overlaps on the shunt signals at the ends of the platforms as opposed to the dead ends.

thanks Dave: that's exactly how I wished I'd put it

Surely the analogy is with ABS on a car- it stops you in the shortest possible distance on both wet and dry roads, but on a wet road that distance is FURTHER. To suggest otherwise breaks the laws of physics and not even MA has proved himself possible of doing that yet has he?

Herr MA does indeed seem to have tied himself in a knot here, mixing up the functions of the antilock braking system with the functions of the ATP speed regulation. What I think he is trying to tell, is that the ATP/ATO and antiskid system on the 92 TS works on similar lines to the antiskid-fitted stocks in Stockholm.

Basically, a restrictive cab signal aspect or ATP code requires the train to reduce it's speed, either by manual braking by the driver, or an automatically triggered braking if the driver is too slow in responding, or if the brake force applied is too weak.

If the braking effort does not come within certain parameters -a certain brake effort applied, an certain amount of speed reduction, within a certain amount of time- the on-board equipment will interpret this as a brake failure and trigger an emergency brake application. The reasoning is that an emergency brake application, while perhaps not as effective as the service brake, will eventually bring the train to a stop, whilst a possibly defective service brake cannot be relied on to do so.

Now, the anti-skid system on a car measures the speed of each motored axle. If the speed of the axle falls outside certain parameters, it is interpreted as either spinning (too high) or locked axles (too low) Depending a bit on the exact type of stock, it might have a feature to lower the traction current to stop the spinning of the wheels, and all stocks have a facility where the brakes are released on wheels that are interpreted as locked.

This means that a train can recieve a restrictive cab signal aspect, the brake force is interpreted as too low, due to slippery track and skidding, and a full service brake application is forced by the ATP system. Since the track is slippery, the anti-skid system reacts when the full service brake applies, and releases the brakes to prevent wheelslide. The ATP system notes that the brake force is still not enough, and applies a full service brake again, which the anti-skid system of course promptly releases. And then the ATP possibly interprets this as a service brake failure and dumps the train into emergency.

This is of course a simplified descriotion, and a worst-case scenario, but it is quite common for us to have to fight our anti-skid systems from time to time.

But, again, please note that the anti-skid system has nothing to do with the ATP system of a train. It is not an ATP function, although it inderectly impacts on the ATP functions by theoretically providing for the best possible adhesion under all circumstances. In an ATO system, the driver's response is of course replaced by the response of on-board electronics.

The effectiveness of the anti-skid system is, of course very much dependant on how the system is constructed, and what parameters are put into it. A well-designed system should ideally keep your wheels just around the point where they might start to slip, since that is the operations zone where you have the largest amount of adhesion.

Modern, sophisticated anti-skid systems often have a feature where, in case of emergency braking, the anti-skid system is active on most axles, but the leading axles on the train are allowed to lock up and skid, in order to act as a scrape, cleaning the track surface and giving better adhesion to the rest of the axles.

Other systems use magnetic track brakes, or even eddy current braking in case of emergency braking. Eddy current braking gives a tremendous amount of braking effort, and is completely independant of adhesion conditions, since it is contactless, but it also has it's drawbacks -expensive, requires power to operate and can actually heat the track enough to damage it. It is also reputed to be able to fry a laptop computer at a repectable distance. Cool!

Rheostatic or regenerative braking does actually depend on adhesion, although in an indirect manner. Electrical braking works by transforming the kinetic energy in a moving train to electrical energy by basically short-circuiting the traction motors and using them as generators. If an axle is locked and does not turn, then no electromagnetic field is generated, and no energy transfer takes place.

Traditional magnetic brakes use a sled or shoe attached to the bogie that is slammed down on the railhead and held there by a powerful electromagnet. This gives a tremendous amount of friction, and thus a very effective,if violent, brake. A drawback is that it is essentially binary -it is either applied, or off. It is quite difficult to construct a magnetic brake that can be "feathered", like an air brake, and thus they are mostly used as an supplemental emergency brake.

Eddy current brakes use a very strong electromagnetic field to grip the rail, but no physical contact. In some ways, you could say that it is an upside-down Maglev train, attractign the train to the track instead of repelling it. It can give an extremely effective brake, enabling very tight spacing between trains a very high speeds.

Bit of a thread revival, this time a question about ATO and fog. Are their any rules as to operating a train in reduced visibility,and if so what would the distances be to change to manual. I could see leaving it in auto mode and keeping close to time, but I would imagine it could be very tense for the T/op

As long as nobody is working on the track, but presumably there wouldn't be if visibility was that bad.

(20 months - is that a record for thread revival? )