The concave vertical space on the side of a rail profile, located between the underside of the rail head and the top of the rail base. This area is designed to hold joint bars snugly against the rail web. A precise fit in this space is required to keep the rail ends aligned and prevent the joint from sagging under heavy axle loads.
Heard on the job
Make sure you clean the rust out of the fishing space before you bolt those joint bars on.