Complex sentence: With the advent of metrication, standard gauge was redefined as 1,435 mm, a reduction of 0.1 mm but well within existing tolerance limits, with the exception of the United States and Canada, where standard gauge continues to be defined in terms of customary units.
Split into simpler sentences: With the advent of metrication, standard gauge was redefined as 1,435 mm, a reduction of 0.1 mm, but well within existing tolerance limits. The exception is the United States, where standard gauge continues to be defined in terms of customary units.