Couplers are used to connect various model railroad train cars together. Couplers are typically either hook or knuckle type. Couplers have three distinct criteria. First, they must join the cars when the cars are pushed against each other. Second, the coupler needs to be able to be uncoupled. Third, from the manufacturer's perspective, the coupler must be inexpensive to produce. Additionally, model railroad train enthusiasts prefer a coupler that looks realistic.

Horn hook couplers are the "standard" couplers usually included with a purchased model railroad train set. Horn hooks are typically included on trains toys. The horn-hook looks like a hook or the handle end of a cane, which joins the cars together. As the model railroad train cars are pushed together, the spring-action of the hooks connects them. The couplers rely on side-to-side pressure to hold the cars together. This can create problems when backing up the train as the pressure is loosened, possibly causing derailment. To uncouple a car, you can either lift it off the track and release the hook or use a device (such as a toothpick) to pop the train cars apart. For the casual model railroad train fan, horn hook couplers are usually sufficient.

Knuckle couplers resemble a person's knuckles and look more realistic when compared to real-life trains. The knuckle couplers join together much as two hands are joined together fingers-to-fingers. This creates a fairly tight fit that will hold the train cars together around curves and in reverse. To uncouple the train cars, you can install magnetic un-couplers. They work by separating train cars as they pass over a magnet mounted under the track. The couplers contain one or two magnetized pins that sit just above track level. When the train is stopped over the magnet and then reversed, the cars become uncoupled. Alternatively, you can disconnect the cars by using a disconnect tool such as the "Switchman." This tool is inserted in the knuckle couplers and rotated to pop the train cars apart. With this tool, you can uncouple train cars from anywhere on the track.

A model train railroad enthusiast might consider converting from a hook-style to a knuckle-style coupler to provide a more realistic and easier-to-operate system. The conversion could be costly as you might need to build shims to keep the pins and couplers at the correct train-to-track height. This is especially important when lining up knuckle-style couplers with magnetic de-couplers. If you have an extra train car, you might consider leaving it with one hook-style end and one knuckle-style end for ease of moving train cars around your track.

For a trains toy, the standard coupler is usually the best choice as children are often not as careful with their toys as a true model railroad fan. However, if your child's interest in his trains toy is growing, you might consider upgrading him to a railroad train model set and teach him how to convert the couplers.
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