If there is an actual physical line on the floor, and you are on
it, then you are on line. If you are in a line of people, you are
in line. If you are in a line of people, and also standing on a
physical line on the floor, then you are both in and on line. If
there is a group of people, standing in a line, and you are
standing on top of the group of people standing in line, then you
are standing on line.