Mostly, an RDC had problems being detected when it rode up on traction sand, thus breaking the track circuit shunt. The cars were too light to crush the sand sufficiently to maintain electrical contact. This was especially a problem when the cars were operated singly, as they were on the WP.

As I recall from my Grandfather's stories, some rule was made about not laying sand for a certain distance before a stop, to minimize this problem. But even so, there were still to some extent, similar issues with the crud and scum that naturally accumulates on the rails; especially on much of the WP, where there were often many hours that the track stood empty, thus not being regularly cleaned by the passage of traffic.

In any event, WP's engineering and operating departments were well of these issues, and did not like the RDCs, and did not trust them to always detect. Thus a non-stop meet between RDCs, would not even be attempted unless the particular siding in question was track circuited (very few were). Unless the DS was certain where the cars were, he would not want to throw any switches. And with a dark siding, there is no way he could be certain until that phone call from the siding told him so.

OPB