This was from back in ~1998, think Mike R was still running TAS then, so may not have detailed recollection on this piece.
From (foggy ) memory, I seem to recall there might have been something relating to adjusting the position of the micro switch that is mounted on the beam. This switch triggers the trolley changing direction after the culvert drops. (or perhaps triggers the direction change, thereby dropping the culvert on the way back, like I said slightly foggy memory )
-Dave