Imagine if you have nothing but two parallel ropes each a mile long, and you're at the midspan of them.  The point is, you don't have access to any of the four ends.  How do you join the two ropes at the midspan without using a lot of rope in some big, bulky, tangled mess?

Back to the problem.  Take the thicker* of the two parallel ropes, if one exists, and shape it into a "U" shape.  Take the other line and make a long bight (double-up the rope) and treat it like the end of a rope and form the "p" or "q" part of the sheet bend.

If you did it right, you should have the two ropes joined at midspan with a little loop at the knot from the bight of the smaller line you were treating like a single rope end.  Now you can pull at any combination of the four ends of the rope.

However, since it's almost always better to hitch to an object if practical, you might just try doubling up the rope and treat it as the end of a rope to tie a timber hitch around the endless rail.