49 CFR § 213.357 - Derails.
(a) Each track, other than a main track, which connects with a Class 7, 8 or 9 main track shall be equipped with a functioning derail of the correct size and type, unless railroad equipment on the track, because of grade characteristics cannot move to foul the main track.
(b) For the purposes of this section, a derail is a device which will physically stop or divert movement of railroad rolling stock or other railroad on-track equipment past the location of the device.
(c) Each derail shall be clearly visible. When in a locked position, a derail shall be free of any lost motion which would prevent it from performing its intended function.
(d) Each derail shall be maintained to function as intended.
(e) Each derail shall be properly installed for the rail to which it is applied.
(f) If a track protected by a derail is occupied by standing railroad rolling stock, the derail shall be in derailing position.
(g) Each derail on a track which is connected to a Class 7, 8 or 9 main track shall be interconnected with the signal system.