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What is a tension lock coupling?

Think back to when you were a child and you acquired your very first 00 gauge train set, complete with a whole oval of track, a tank engine, up to as many as three wagons and a controller. It was probably one of the best days of your life. Now think about the couplings that adorned the much loved and admired model train and that was most likely a tension lock coupling!
The truth is, even if you can't relate, you would probably still recognise them because in the UK most ready to run models still use them. They are the big D shaped piece of moulded plastic on the front of your engine with a hook protruding from it that latches onto the big D shaped piece of moulded plastic on the opposing rolling stock. They may have a slightly more streamlined appearance but they are still the good old tension lock coupling.
We still use them today for two reasons: firstly, they work, they may not be everyone's cup of tea but they do a good enough job even if they don't look completely prototypical. Secondly, most modellers have an abundance of older models with tension lock couplings, when they purchase new models usually we would like them to be compatible with our existing stock so manufacturers are reluctant to change them as standard. They do nowadays though fit detachable NEM tension lock couplings so that those who desire it can easily change the couplings for different designs.
Tension lock couplings work by loosely hooking over the opposing D, once the slack between the coaches is taken up by the forward motion of the train the connection becomes more solid due to the special shape of the D's profile marrying with the shape of the hook, this allows horizontal movement but not vertical hence the term tension lock.
As mentioned tension locks are not for everyone, but most see them as an acceptable compromise probably because we have lived with them for so long, so in conclusion, we think they will be around for some time to come.
Posted in: Model Railway

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