Those are interesting, especially the Global Review article that emphasises the importance of breathability and waterproofing, but I couldn't find anything in them that says why orange is specified rather than yellow.
The reason why since about the 1960s orange was used as personnel hi vis was because as train speeds increased the traditional white over clothing or arm bands was completely inaffective, let alone mucky grey/brown/black shirts etc, train drivers simply couldn't see track workers far enough away to stop in anything other than good weather conditions. Although the world steam speed record was set in 1938, by and large the majority of trains travelled at only 60mph except at a few mainline locations. But then modern high speed diesel locomotives and electric trains which were inherently quieter, were rolled out in the 60s capable of sustained running at 80 or 100mph, resulting in much greater risk to trackside workers who might not hear them until it was too late to move clear. It was also when diesel and electric locomotives started to have their noses painted yellow to help workers spot them from a distance. Lookouts at work sites use flags to indicate to drivers, green = clear, yellow =caution and red = effing well stop. Drivers are trained to respond to these colours and so a worker wearing yellow even 10 feet from the line could solicit an immediate precautionary application of the brakes. Orange isn't used as a flag colour so a train driver seeing orange near the line but not on, could proceed at speed subject to any other indication. If you look at any railway worksite you will see that any areas cordoned off with temporary plastic fencing is always blue, never yellow or orange. Again to avoid confusion and to give contrast to workers who might be lineside and haven't seen the approaching train but might suddenly move onto the four foot.