The circuits shall be so installed that each signal governing train movements into a block will display its most restrictive aspect when any of the following conditions obtain within the block:

(a) Occupancy by a train, locomotive, or car,

(b) When points of a switch are not closed in proper position,

(c) When an independently operated fouling point derail equipped with switch circuit controller is not in derailing position,

(d) When a track relay is in de-energized position or a device which functions as a track relay is in its most restrictive state; or when signal control circuit is deenergized.