49 C.F.R. § 218.23 – Blue signal display.

(a) Blue signals displayed in accordance with § 218.25, 218.27, or 218.29 signify that workers are on, under, or between rolling equipment. When so displayed—
1. (1) The equipment may not be coupled to;
2. (2) The equipment may not be moved, except as provided for in § 218.29;
3. (3) Other rolling equipment may not be placed on the same track so as to reduce or block the view of a blue signal, except as provided for in § 218.29 (a), (b) and (c); and
4. (4) Rolling equipment may not pass a displayed blue signal.
(b) Blue signals must be displayed in accordance with § 218.25, 218.27, or 218.29 by each craft or group of workers prior to their going on, under, or between rolling equipment and may only be removed by the same craft or group that displayed them.