We have all taken a paperclip and bend it a number of times till you break it. For those that have not heard how that is done, the metal at the bending is being work hardened as you flex it, until it becomes brittle and then it has no more strength and it breaks very easily.
When train run over a track, the rail is bent slightly but after a while of a large number of years, it is work hardened and it will become brittle just the same as the paperclip. The temperature change from winter to summer also work hardens the rail even without any trains flexing it. Smaller rail will flex more than heavy strong rail, so therefore after, say, 100 years some 70# rail will become so brittle that it will break at any time, especially during some cold spell.
One of the few advantage of the smaller light rail is that when it does become traffic harden, but before it becomes brittle, it will not wear out on the surface and it will handle extreme cold weather better than larger rails. Tiny flaws in rails larger than 90# have more habit of developing to a break in the rails extreme temperatures below zero.
New rail is very soft metal and if under heavy traffic it will have metal flow, corrugation of the surface, and other defects develop before it becomes work hardened. New made hardened rail is very expensive and only put on sharp curves and special trouble spots.
I would rate the C&TS as having very good traffic-hardened (valuable) rail that has not reached anything close to brittle stage yet, as I do not hear of many broken rails being found daily, or weekly. And the surface head of the rail is generally very fine without corrugation. You would find many broken rails after a track surface job that has just occurred on the C&TS if the rails were brittle. Brittle rail does not like its sub-base changed and will fail.
