A horse that cannot overtake another horse because it is blocked by other horses.
A horse that is racing on the rails (or fence) and is surrounded by other horses in front, outside and behind it. A horse that is boxed in is held up and unable to gain a clear passage.
When a horse is racing along the pylons and cannot improve his position in a race because of the presence of other horses in front, behind and beside him. Also called Locked in.