Yesterday there was an electrical problem that left me completely baffled and it has taken me 24 hours to discover the underlying cause.
The problem concerned electrical power to the down GWR lines. Trains had been running normally but suddenly I saw the down train slow and stop. I tried to cajole it to restart but it was not interested in running normally on any part of the lines.
I swapped locomotive but that did not help. That made me suspect the controller and so I swapped controllers. It didn’t help when using the ‘up’ controller. I decided to try running a locomotive along the station straight (an isolated section of track) but using a pair of new wires from the controller and applied directly to the rails. This did not overcome the problem.
I stopped investigation yesterday at that point – deciding to sleep on the problem. Today I started from basics and used a new, separate short section of track sitting on the station platform and attached to the controller via the pair of wires. The locomotive ran OK in this configuration but not when transferred back to the layout. So I knew the problem lay with the track layout.
I turned my attention to a set of points feeding the station lines, and I isolated the power feeding through those points by inserting wooden cocktail sticks to prevent the point blades from making contact to the rails. This did the trick and the locomotive ran around 3/4 of the layout but then suffered a similar problem towards the end of the fiddle yard. I did the same trick with the exit set of points and the locomotive was fine.
It was at this stage that I noticed a set of carriages in one fiddle yard lane had rolled forwards and too near the end of its isolated section of track. I pushed it back to its normal position and that completely fixed the problem.
I now understand the electrical fault. The front set of carriage wheels had bridged the isolation gap in the rails. When the points upstream of this position were switched for the running of the locomotive this action triggered a polarity change in the rails and a short circuit was generated.