Complex sentence: Once the Cape Colony had finished building a railway line to Johannesburg, it attempted to get as much of the Transvaal's railway traffic as possible by reducing its rates, as it was aware that the Transvaal's Delagoa Bay line was almost complete.
Split into simpler sentences: Once the Cape Colony had finished building a railway line to Johannesburg and attempted to get as much of the Transvaal's railway traffic by reducing its rates. It was aware that the Transvaal's Delagoa Bay line was almost complete.