It is my understanding that 'A' railroad builds a track north-south. 'B' railroad builds east-west but has to cross over 'A's railroad. Rather then have the expense of going under or over 'A's track, 'B' railroad negotiates a deal with 'A' railroad that allows 'B's track to cross at grade. The two railroads then maintain the diamond jointly, though 'B' railroad may have to pay more of the maintenance expense. Both railroads maintain their own track up to the diamond.

My assumption says, 'A' railroad builds the tower to look over and dispatch their railroad and allows 'B' railroad to cross only when there is a break in 'A' roads action.
Yes? No?