The worry here is that in figure 7 the distance between C5 and C6 is shown to be ½ meter. The distance between C8 and C9 is 1.88 meter (for freight car identification). Thus we can assume that C8 and C9 is the wheel spacing for adjacent Freight Cars where as the locomotive wheel axle spacing (bogie) is ½ meter. In this sense D1 is recognizing mechanically the part to be shielded.