Complex sentence: Modernisation further took place under the Austrian Empire and the Brenner Railway, which was completed in stages from 1853 to 1867, became the first trans-Alpine rail route being built without a major tunnel and at high altitude (crossing Brenner Pass at 1,371 m).
Split into simpler sentences: Modernisation further took place under the Austrian Empire and the Brenner Railway, which was completed in stages from 1853 to 1867. It became the first trans-Alpine rail route being built without a major tunnel and at high altitude (crossing Brenner Pass at 1,371 m).