While rulebooks had codes for conductors communication signals on passenger trains (e.g. one buzz - or whistle in the old days - meant stop movement) I'm not aware of any codes intended for the attendant call system on locomotives. These types of systems were not able to communicate between locomotives and cabooses on freight trains - such communication was by radio, or hand / lantern signals. UP used to have very long fusees with yellow flames that were used at night and for steam locomotives, the steam from the whistle could be observed.