Definition of "metals"

Now and again a night train roared along the metals within twenty feet of him; but he had all the Oriental’s indifference to mere noise, and it did not even weave a dream through his slumber.

1900 December – 1901 October, Rudyard Kipling, “chapter 8”, in Kim (Macmillan’s Colonial Library; no. 414), London: Macmillan and Co., published 1901

They stood on the foot-bridge over Boathouse Station and looked at the metals gleaming coldly.

1913, D[avid] H[erbert] Lawrence, “chapter 7”, in Sons and Lovers, London: Duckworth & Co. […]

Steam services began running over NR [Network Rail] metals from Grosmont to the traffic objective of Whitby in 2007, with operations enhanced by opening of the second platform at the terminus in 2014.

2019 October, James Abbott, “Esk Valley revival”, in Modern Railways, pages 77, 78