One of the most dangerous times for a train is when braking just as a rain shower starts. The light sprinkling of water on the head of the rail mixes with the oil and grease resulting from passing trains, atmospheric pollution and flange greasing and it produces a lethal surface similar to that of an ice rink. As soon as the driver touches the brakes, the wheels lock and slide along the rails. The speedometer in the cab suddenly drops from 50 mph to 0 and you know there is nothing you can do. If you apply the emergency brake, more wheels lock further along the train and the slide just damages more wheels. If you release the brake, you won’t stop where you are supposed to. If you let things stay as they are, you won’t stop where you are supposed to anyway. You might just as well “drop the lot” and let the train stop where it will. At least everyone will hear all the air escaping from the brake pipe and see that at least you tried. The fact that the whole train set of wheels will have developed flats is not your concern at the moment (so sue me!).
Another bad time for sliding is early in the morning when the first train has to run through all the dew which has collected on the rail head overnight. Great care is needed to get the train from one end of the line to the other without overrunning at least one station. The same applies during the leaf fall season (as railways call autumn nowadays), when the slush of squashed leaves mixes with rain and oil to form a nice sliding mess on the rail. Of course, if there is heavy rain, the conditions are better because the water washes the rail head to some extent and reduces the risk of a slide.
Why do we get all these problems now? We never heard of “leaves on the line” in the 1950s and 60s. The answer is progress. As modern trains have got lighter, the reduced weight has reduced the adhesion available, so a slide becomes easier. Another reason is the change from cast iron brake blocks to composition blocks. Cast iron is heavy (they used 28 pounders when I had to change them) and it produces an inflammable dust which plays havoc on electrical equipment on the train and inside signalling equipment cabinets. A third reason is the shift from tread brakes to disc brakes. Tread brakes provide a nice scrubbing action on the wheel surfaces. Disc brakes don’t touch the wheel so the tread gets dirty and oily and slippery - first class slide material.
How can the problem be solved? A good question which has not yet been answered. There have been many attempts to give slide protection by automatically (i.e. without any action by the driver) releasing the brake on any wheel which starts to slide - not much better than letting the train slide, except that it does reduce flats. However, it does nothing to help the driver approaching closed level crossing gates as one discovered one night in south west London a few years ago. He demolished a Volkswagen minibus, if I remember correctly. Fortunately, no one was killed.
Another solution is - just that, a solution - of gunge put on the rails to clean them. Sandite is a popular brand in the UK, but it requires a special train to do it and a crew to crew it and it cannot be everywhere at once. Of course it is never there when you need it - first thing on a November morning between Rickmansworth and Chalfont, for example.
One final point - how do automatically operated trains cope with bad rail conditions? Well, most introduce a lower speed during wet or icy weather (e.g Singapore Mass Rapid Transit in a rain storm), some increase the braking distances on open sections of the line (London, Central Line) and some even turn over the driving to the drivers so that they can use their judgement to ensure that the train stops in the right places. It was the removal of such a procedure by a new manager which caused an accident a few years ago on the Washington DC metro (WMATA), in which a driver was killed.
Good informative post. Thanks!