I think it is the author applying imagery notice the SHrill, ruSHing, SteamwhiStle, it is implying the sound and image that of a train, and the SteamwhiStle is to imply that of the steam coming from the train. now the r in RuSHing is to imply the speed or power of the locomotive.
The sounds S and SH among closely connected words, called alliteration, are intended to resemble or simulate the different sounds of the locomotives as the sentence is read aloud. Thus, rush is a quick movement; roar is deep, long animal cry; shrill is a high-pitched voice or sound; and steam whistle is a high-pitched sound which forces steam out of a small hole.