33 CFR § 117.389 - Calumet River.
The draws of the Norfolk Southern railroad bridges, miles 1.32 and 1.36 at Chicago, operate as follows:
(b) In addition to the signals prescribed in § 117.15, the following special visual signals shall be used on the bridges:
(1) When the draw cannot be opened immediately, or is open and must be closed promptly, two red lights are flashed alternately.
(2) When the draw can be opened immediately, two amber lights are flashed alternately.
(3) When the draw is open for passage, two green lights are flashed alternately.