signals can only be passed upon special permission from a control authority. Cab Signaling Signals - Where cab signaling is employed without fixed trackside automatic signals, special signal aspects are required at absolute signals. These include some sort of absolute block "Super Clear" signal that allows passage to the next interlocking with a fixed signal and also the "Cab Speed" signal that informs the engineer to proceed under direction of cab signals. Distant (approach) signals Distant signals are often referred to as approach signals as the signal block before the interlocking is known as the approach block. In the aftermath of the 1996 Silver Spring Collision, the Federal Railroad Administration amended its regulations for push-pull train operation to prevent locomotive engineers from forgetting that they were approaching a stop signal after making a station stop. The resulting "Delay in Block Rule" requires that all distant signals, located in territory where push-pull trains operate in the absence of cab signals, be marked with a "D" placard. The placard is intended to remind engineers that they are bound by a speed restriction, and must approach the interlocking signal prepared to stop, whenever a station stop is made or train speed drops below in the approach block. The restrictions hold until the interlocking signal is clearly visible and is displaying a "proceed" indication. References Sources Armstrong, John (1957). "All About Signals." Two-article series. Trains Magazine, June and July 1957. "Canadian Railway Operating Rules - CROR Signal Aspects." 2010-02-01. Pennsylvania Railroad (1968). "Rules for conducting transportation." 1968-04-28 External links Railroad Signaling and Communications - photos and info Mobile CROR Signal App. Can be used as a tool to help learn Canadian Railroad signals. Rail transportation in North America Railway signaling in Canada Railway signaling in Mexico Railway signaling in the United States