Rail horses in America aren't required to do much. Walk trot canter, reverse, stand quietly or walk jog lope, reverse, stand quietly. Depending on the class, they may ask for extended trot or hand gallop. Gaited horses obviously get asked for their respective gait.

So, once the horse knows the aides and is socialized and travels in the correct frame thay are considered finished after a few shows. It is really easy to ring sour a rail horse from repetition. So, I think drilling and drilling is counter productive.

With Dressage there is always something else to learn and practice.