Changes

In order to prevent trains from colliding, railroads use electrical signaling systems.  The most common systems in use today are based on variations of [[automatic block signaling]], which can be supplemented by [[positive train control]].

In order to prevent trains from colliding, railroads use electrical signaling systems.  The most common systems in use today are based on variations of [[automatic block signaling]], which can be supplemented by [[positive train control]].

==== automatic block signaling ====

==== Automatic block signaling ====

In automatic block signaling, railroad tracks are divided into sections known as blocks.  The signaling system prevents more than one train from occupying any one block at any given time by detecting the presence of trains and by using colored lights (similar to [[traffic signal]]s) to indicate to train crewmembers whether it is safe to proceed<ref name="train control systems">''See'' [http://www.ntsb.gov/events/symp_ptc/presentations/03_VanderClute.pdf Train Control Systems].</ref>; for example, a yellow signal may be used to instruct train crews to slow down, and a red signal may be used to instruct train crews to stop so as to avoid entering a block that is occupied.

In automatic block signaling, railroad tracks are divided into sections known as blocks.  The signaling system prevents more than one train from occupying any one block at any given time by detecting the presence of trains and by using colored lights (similar to [[traffic signal]]s) to indicate to train crew whether it is safe to proceed<ref name="train control systems">''See'' [http://www.ntsb.gov/events/symp_ptc/presentations/03_VanderClute.pdf Train Control Systems].</ref>; for example, a yellow signal may be used to instruct train crews to slow down, and a red signal may be used to instruct train crews to stop so as to avoid entering a block that is occupied.

==== positive train control ====

==== Positive train control ====

[[Positive train control]] is a technology by which "automatic control systems" are able "to override mistakes made by human operators" in order to prevent train collisions.<ref name="NTSB recommendation">http://www.ntsb.gov/recs/mostwanted/positive_train.htm</ref>  [[Amtrak]]'s Northeast Corridor, for example, uses a system that comprises two elements, "positive train stop" and "civil speed enforcement," which together provide the functionality of positive train control.<ref name="NTSB recommendation" />

[[Positive train control]] is a technology by which "automatic control systems" are able "to override mistakes made by human operators" in order to prevent train collisions.<ref name="NTSB recommendation">http://www.ntsb.gov/recs/mostwanted/positive_train.htm</ref>  [[Amtrak]]'s Northeast Corridor, for example, uses a system that comprises two elements, "positive train stop" and "civil speed enforcement," which together provide the functionality of positive train control.<ref name="NTSB recommendation" />