49 C.F.R. § 234.309 – ENS signs in general.

(a) Provision of information. If the dispatching railroad and the maintaining railroad(s) are not the same entity, the dispatching railroad for a highway-rail or pathway grade crossing shall provide to the maintaining railroad the telephone number that is to be displayed on the ENS sign at the crossing, not later than 180 calendar days before the date that implementation of an ENS is required.
(b) Information to be displayed. Each ENS sign located at each highway-rail or pathway grade crossing as required by § 234.311 shall display the necessary information for the dispatching railroad to receive reports of unsafe conditions at the crossing. This information, at a minimum, includes the following:
1. (1) The toll-free telephone number (or non-toll-free telephone number as provided for in § 234.303(e)) established to receive reports pursuant to § 234.303(a);
2. (2) An explanation of the purpose of the sign (e.g., “Report emergency or problem to __”); and
3. (3) The U.S. DOT National Crossing Inventory number assigned to that crossing.
(c) Sign size and other physical features. Each ENS sign shall—
1. (1) Measure at least 12 inches wide by 9 inches high;
2. (2) Be retroreflective;
3. (3) Have legible text (i.e., letters and numerals) with a minimum character height of 1 inch for the information required in paragraph (b) of this section; and
4. (4) Have white text set on a blue background with a white border, except that the U.S. DOT National Crossing Inventory number may be black text set on a white rectangular background.