Does anyone know references about that topic: The dark colour of the rail aggregate might provide thermal benefit to amphibians (or newts), accelerating their metabolism.
The book Herpetology: An Introductory Biology of Amphibians and Reptiles by Vitt and Caldwell has a section on thermoregulation, performance and energetics, and discusses the influence of dark surfaces/substrates on heat transfer (although not specifically mentioning the dark colour of rail aggregate it would still be relevant to your topic I believe). Hope this helps!