The main reason it's so expensive to convert European standard gauge equipment to knuckle couplers is that, traditionally, it had no center sill. All compression forces (buff) went through the side sills, with the buffers at the end of the sills. A heavy end sill (buffer beam) connected the tension forces to the side sills. Knuckle couplers, on the other hand, are designed to transfer both forces to the center sill, and require extra bracing to use the side sills. This is a factor in Center Flow covered hoppers and double stack well cars.
Most European narrow gauge, and the traditional North American link and pin coupler, use a center buffer of some type, so conversions to knuckle couplers are relatively easy.
In a somewhat related vein, I once saw a video, from Switzerland, of what looked like an automatic link and pin coupler. Instead of a standard link socket, it had a funnel-like socket, and the link on one car would be guided into place in the other car by the funnel. The pin was replaced by a sprung (gravity?) catch, which would move up to allow the link in, then drop to capture it. Uncoupling used a lever on the end of the car, much like that for knuckle couplers. It looked like a very simple design that could have been used in North America. If neither car had a link, one could be inserted by hand, before the cars were brought together. Much safer, and easier, than the traditional way.
Also, the screw-link couplers aren't nearly as dangerous as they look. Since the ends of the cars never touch, and the cars can be brought together very gently, it's actually safe to stand between the rails during coupling. Once the buffers touch, pick up the link from one car, drop it over the hook on the other, and tighten. Then duck under the buffers and give the driver the signal to move. Not like trying to hold a link in place while the link socket/buffer blocks come together. The biggest problem with screw-links is the limited tonnage, and European railroads tend to run shorter, more frequent freights, to fit into the passenger oriented scheduling.
