Complex sentence: The Leeds and Thirsk had changed its name to the Leeds Northern Railway before it opened a line to Stockton, formed an alliance with the West Hartlepool Harbour & Railway and became involved in a price war with the York, Newcastle & Berwick Railway (YN&BR).
Split into simpler sentences: The Leeds and Thirsk Railway Company changed its name to the Leeds Northern Railway before it opened its line to Stockton. It formed an alliance with the West Hartlepool Harbour & Railway and was involved in a price war with the York, Newcastle & Berwick Railway (YN&BR).