Complex sentence: A train conveying flasks would be hauled by two locomotives, either Class 20 or Class 37, but Class 66 locomotives are increasingly being used; locomotives are used in pairs as a precaution in case one fails en route.
Split into simpler sentences: A train conveying flasks would be hauled by two locomotives, either Class 20 or Class 37, but Class 66 locomotives are increasingly being used. Locomotives are used in pairs as a precaution in case one fails en route.