Ruling gradient

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The steepest sustained grade on a section of line, and the number that governs everything else about it. Because a train must be able to climb the worst piece of the route unaided, the ruling gradient fixes the minimum locomotive power or the maximum train weight for the whole run — the weakest-link property that makes one hill the arbiter of a thousand kilometres of flat track. Railways buy their way out of it in only three ways: more power on the front, helper locomotives stationed at the hill, or civil engineering that removes the grade. The Canadian Pacific tried all three at Kicking Horse Pass, and the last one — the Spiral Tunnels, taking 4.4 to 4.5 per cent down to 2.2 — is the one that stuck. This is why alignment is the expensive, permanent part of a railway and rolling stock is the cheap, replaceable part.

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