My guess is it is placed so as to protect whatever is behind it from a possible runaway
car, thus preventing a far greater hassle than re-railing the runaway. Please correct
me if I'm wrong.
Hoppy
Original Post
Replies sorted oldest to newest
Exactly.
Or prevent cars on a siding from entering the main.
it is used to protect the main track from a car rolling out of an industry or out of a siding through a switch and out on the main which is always a bad thing.
Simply put, a derail is to prevent a railroad car or locomotive from being were it shouldn't be until it is removed from the rail .
Derails are located on service tracks protected by a "blue flag",in sidings off a mainline,sometimes protected by a switch with a timer,or an industry track after you enter from the mainline.
A few years back the NS here on the Kenova District had pipe connected derails that when you threw the switch,pulled the derail off the rail.
And just about two years ago the Kenova District removed it's mainline derails that were referred to as an "alligator" derail.It reminded you of a switch to no where from double track to single track.And their purpose was to stop a head-on when a train ran by a stop signal when an approaching train took a favorable single coming from the other direction.
And trust me,not by personal experience thank goodness,but they were very effective 
