Hook and loopcouplers are one of the many non-prototypical but common styles of model train couplers. Hook and loops are, as the name implies, comprised of a plastic or metal loop and hook. In most versions, the hook is sprung and engages with the loop on the neighboring car. The large loops allow trains to negotiate tight curves without making the couplers themselves moveable.
Although some similar designs can be found on small-gauge industrial prototype roads, hook and loop couplers are better suited for models. Coupling is automatic and there is no need to center the couplers. Uncoupling can be more difficult. Some designs allow the use of a mechanical ramp to disengage the coupler. In G scale, hook and loops can be easily uncoupled by simply pressing a popsicle stick or similar object down between them. They are well suited for children.
Hook and loops have been common on model trains for years. Lionel used a version on its models in the early post-war period. Mantua is probably the best known example in HO scale. Today the most frequent use is in G scale on products offered by LGB and others. Although each of these followed the same principles, they were all unique designs.