From reading the description, it sounds like a stress fracture, and if so it may have nothing to do with the quality of steel. If a replacement shows the same kind of crack in the same spot, that would indicate to me that maybe the roadbed has some sort of softness in it or unevenness, that is putting strain on the rails. Cracks like that can happen from torsion or tension, and given that both the old rail and the new one apparently did the same thing, likely.
As far as the quality of Chinese metal products, time will tell about the, the replacement bay bridge is Chinese steel. I can say that with hardware, the quality of chinese made hardware is pretty bad usually, stripping the head on a screw is so common I buy a lot more screws and such when doing projects then I would have not many years ago (I generally go where I can buy stuff not made in China, I pay a bit more, but have a lot less losses). Doesn't mean that you get better quality in everything not made there, lot of crap is produced all around the world, but with stuff made there you generally get what you pay for IME, if you are buying based on cheap prices. I have bought some high quality hardware that was made in China, and it was fine *shrug*