Roll off rail length

Whats the set up on the truck? Single axle or tandem? how much over hang is there from the rear wheel to the end of the rail? Is there an under ride bumper there or not? If the rail ends right at the back wheel then you could get away with a 2 foot over hang. If the rail extends several feet past the rear wheel, then the extra 2 feet will possibly be illegal with out proper under ride bumpers and reflectors. Also if there is too much weight past the rear axle, you may run into an under weighted front axle and or over weight rear axle. Again knowing the trucks configuration will better help in answering your question.

In a broad sense though, yes you could haul a 22ft bin on a 20ft rail. Whether it is legal is determined by configuration.

 

Trash bins are never uniform.