Bob, All:
Just a long distance guess, but those fractures look to be classic metal embrittlement failures of the brass. Some factors that can cause this are improper choice of the alloy, inclusions of various contaminants during manufacture, temperature change, and high stress. For general information, see:
Sci-Tech Dictionary: Embrittlement (link)
Usually this type failure is caused by some combination of the above factors. Since this rail is most likely made in China by the lowest bidder, it could easily have specification and / or quality control issues that are not prevalent in western factories. Note that Greg’s “. . . crystallization” suggestion is one failure mode in embrittlement.
Stress, coupled with temperature changes can greatly exacerbate this condition, thus leading to the failure(s) shown above. The fact that the rail straightened out when it broke shows that it was under a significant amount of stress. In this case the stress was most likely provided by just forcing the track into position and screwing it down rather than bending it to shape and installing it in a relaxed condition. Of course, if the rail material all has significant defects, bending it could cause more failures, but they should happen when it is being worked, rather than during temperature changes.
On another front, I’ve found that putting a screw though the middle of a tie often causes failure of the tie. This failure of the plastic is also exacerbated by leaving stress on the rail / tie assembly. In the few cases where I need to fasten track to wood, I bend it close to the desired shape, then use smaller fasteners (screws or nails) at the outer ends of the ties. This also takes force off the simulated spike heads, helping eliminate another possible failure point.
Repairs look to be relatively simple, but I would really suggest you prebend the track.
Happy RRing,
Jerry