Well, that’s what you tell us when you get caught out trying to sneak across the river on a bank holiday when only one turn out of knip! 
They still use yellow flashers to signal warnings. Admittedly, I can’t seem to find any references for such. I remember riding the school bus when younger, and always seeing the reminder sign telling drivers that they were prohibited from using the red flashers within the city limits. Or maybe out of the city limits. One or the other; that was a while ago. While looking at references, though, there are a lot of specific spots (as opposed to general, e.g., a whole city) where the red flashers are prohibited, mosty to do with whether kids need to cross the street or whether there are traffic control devices present (e.g., no red lights if the stop is controlled by a cross walk with pedestrian signal). I’ve also spotted a reference to where I think a school bus near my house is inappropriately using its red lights, but I’m not one to complain about it. I liked finding this one (257.1855): “Before resuming motion, the driver shall deactivate these lights and allow congested traffic to disperse where practicable. The deactivation of these lights is the signal for stopped traffic to proceed.” Crankshaft be damned. Also of note, it is legal to pass the school bus with red flashers in lots of situations, notably, while the bus is in motion.
Back to rail road crossings, anyone know the general rules for trucks? I often see trucks labeled as “This vehicle stops at all RR crossings.” Often they’re tankers of some sort, although I see the occasional box-type trailer with the advertisement. Is this a company policy, or are there federal/state regulations that demand this for certain types of cargo?
Seems pretty hit and miss to me. Most of the unprotected crossings I use definitely don’t have stop signs. Some, though, do.
In Mexico it always strikes me as odd that they pretty much stop at all railroad crossings. Nothing like doing 140 (k/h, that is) on the highway, and having to suddenly come to a skreetching halt. On the other hand, with their reputation for maintenance, it seems like the logical thing to do whether it’s the stated law or not!
While it may be a company policy, it is typically determined by the commodity being transported. State and Federal regulations cover transportation of hazardous materials as defined in 49 CFR 171-180. One must possess a CDL (Commercial Driver License) with appropriate endorsements to transport hazmats by truck, trailer, intermodal, or tanker. Vehicles will be placarded based upon the hazard class of the material being transported, and the quantity of product, singularly, or when grouped with others. The short rule is to stop at all railroad crossings if your vehicle is placarded, carries any amount of chlorine, or has cargo tanks used to transport hazmats, whether loaded or empty.
Rules can change, which is partly why we must be retested every four years to retain a hazmat endorsement on a CDL.