Code of Federal Regulations Title 49. Transportation § 49.236.205 Signal control circuits;  requirements

(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.