If a train is too big to fit on a siding, you cut the train for what will fit on the siding and leave the rest out there. Then the other train that is desiring to pass will couple on the the tail end and pull that back to the siding and go around it, hoping you do not need to do a double saw by. Then the first train backs up to retrieve its rear end while the passing train departs.
It can get interesting when the power on one is not strong enough to pull all that must be moved. Some cars can be left in the clear before the siding while it pulls the rear in. Also, the original first train engine can back and couple the entire consists of both trains together to pull that real into the clear, where the passing train can get around and depart. The first train can couple up, test the brakes, and proceed. Lots of fun. Just think of two 100 car trains saw bying each other with only a 35 car passing siding. It can be done.