Coupling Achilles heel

I had noted that some modellers had reported derailments on curved track. During testrunning on my own layout, the third car derailed three times at the same spot. There were no further problems until I tried a test run on the Model Rail test track. Here it ran without derailment on the outside circuit with the widest radius curves but derailed as soon as it was diverted over pointwork. I then took the bodies off a couple of cars in order to examine the interiors. When I next ran the unit at home it would not complete a circuit without at least one of the unpowered cars derailing. I checked that the car bodies I’d removed had been refitted precisely. I then conducted a test-run with car 813004 minus its body. This showed that at the point of derailment the car was actually suspended, off the rails, by the couplings at either end, thanks to a very slight sag in the baseboard. No other train has, however, derailed at this point. The couplings are in the style of an NEM pocket turned on its side. Coupling two cars together provides a rigid bar 56mm long. Each part of the bar is engaged in a close coupling slot which allows it to open out on curves by 5mm per car, giving a rigid bar 66mm long when fully opened out. Within that 66mm there is little or no vertical flexibility. To be fair, we should expect our track to be well-laid and in tip-top condition if we are running fast trains. In practice, we all know the shortcomings of our own layouts and here it’s necessary to decide whether we fettle our track to perfection or whether a minor tweak to the model will resolve the issue. Initially, I felt that a very close interface between the yaw-damper arm on the bogie and the associated bracket on the body might be the problem. Checking that the body has been fitted precisely and that the clips are fully engaged should resolve this. Having decided that a lack of free movement and vertical flexibility in the couplings might be the problem, I slackened each of the coupler screws by no more than one full turn. On four of the cars it is necessary to remove the toilet retention tank mouldings (held by two screws) in order to access the coupling screw. As I completed each car I tested it. I had no further derailment problems, the close couplers move freely, and I can run the unit at close to full speed round my 3ft radius curves.