Intermittent Earths are a bitch to find.
Back in my Central Line days with London Underground in the mid 1990s we had an intermittent earth that screwed up the signalling for about a month. As it was at Marble Arch it caused havoc with the service. Always used to happen in the AM peaks.
The whole area was cleared of any scrap metal etc, engineers on site overnight, nothing found. Points on the crossover given a thorough going over etc.
Eventually the cause was found - there had been a broken chair screw (a screw that holds to the sleeper the thing that holds the rail) in the past that had snapped below the surface of the sleeper. A new screw had been wound in on top of the old one - pushing it out of the bottom of the sleeper. Over time, the track bed had settled a little, thus meaning that the old screw, only under the weight of a fully laden train (hence the AM peak), just made contact with the iron tunnel segment. As the old screw was in contact with the new screw, that in contact with the chair, and the chair in contact with the rail - through which the current controlling the signalling runs, Bingo, a lovely hidden intermittent earth.
Biting the hand that feeds IT
