Complex sentence: The gauge was that used for the early wagon-ways, which had been adopted for the Stockton and Darlington Railway, with a width which became known as the international standard gauge, still used by about 60% of the world's railways.
Split into simpler sentences: The gauge was that used for the early wagon-ways and had been adopted for the Stockton and Darlington Railway. The width became known as the international standard gauge, used by about 60% of the world's railways.