Complex sentence: The history of the London Underground started in the 19th century with the construction of the Metropolitan Railway, the world's first underground railway, that opened in 1863 using gas-lit wooden carriages hauled by steam locomotives.
Split into simpler sentences: The history of the London Underground started in the 19th century with the construction and opening of the Metropolitan Railway. The world's first underground railway, this opened in 1863 using gas-lit wooden carriages hauled by steam locomotives.