It does get confusing trying to figure out was was historical or common practice, and when or if it became an ICC/FRA regulation. The railroads have struggled for years with the liability over grade crossing accidents. Even SP made aluminum the standard for smokebox fronts (below the cab window on cab forwards) in 1946. The first Baldwin and GE diesels were delivered in the standard black with orange tiger stripes used on switch engines, but by mid 1949 this was changed to aluminum ends to improve visibility. I was reading a SP timetable the other day (from early 1970s) and even at that time it stated that it was not a requirement to have the gyralights on except at night or in bad weather.
Do remember that that even if it isn't an FRA regulation, if a railroad has a written operating rule then it is a FRA violation for anyone on the railroad not to follow the rule.
Personally I prefer to be as visible (and load) as possible on a train, but historically headlights were not common in daylight hours on steam or early diesel. Incofer (Costa Rica 42") still runs trains without headlights and just an obnoxious single chime horn, but doesn't seem to have any worse record at grade crossings than the US. Maybe they are a little smarter or lawsuits don't prevail?