Bad Couplers

Yesterday I was running my trains and had a derailment. It occured on a transition from a straight section to a 9 3/4 in. curve was the site of the derailment. The GP at the head of the train had entered the curve, while the box car behind it was still on the straight section. The box car's front truck jumped the rail and the car uncoupled.


Further investigation revealed that it was the combonation of couplers that caused the derailment, specifically the body mounted coupler on my GP-50 and an truck mounted coupler upgraded to MTL couplers using MTL 1128 Coupler upgrade kit. The gP had a 6.5 cm wheelbase and a body size of 11 cm which gives the body mounted couplers a large arc. The long shank coupler on the GP normally provides enough lateral play to negotiate the sharp transistions with another normal coupler. However the converted coupler had a restricted range of lateral motion, practically zero. So this combonation caused both couplers had reached the limits of their lateral motion and began to apply lateral force against each other. This caused the truck of the box car to jump the rail.

The image with this post shows the two couplers in thier centered position at the position of the derailment. The space between the couplers give you an idea how much lateral motion is required to round the sharp curve.

This was not a problem with my other engine since it had a shorter wheel base and body ( 5 cm and 8.5 cm ) The other cars had factory installed couplers so they had enough lateral motion to negotiate the transition with the GP.

As a solution I tried to remount the couplers on the GP slighly closer to the wheels so that thier swing was reduced. This did not help. Since it is only the combonation of the one car and engine, it should be avoidable by not having them coupled while traversing the tight curves. It could also be eliminated by using easements and larger radii curves.