Complex sentence: The LPTB embarked on a massive capital investment programme that extended services and reconstructed many existing assets, mostly under the umbrella of the 1935-1940 "New Works Programme".
Split into simpler sentences: The LPTB embarked on a massive capital investment programme that not only extended services, but also reconstructed many existing assets. This mostly came under the umbrella of the 1935-1940 "New Works Programme".