Complex sentence: The line is only electrified from London Paddington to Airport Junction, which allows the Heathrow Express and Heathrow Connect trains to operate and this electrification will be extended as far as as part of the Crossrail project.
Split into simpler sentences: The line is electrified only between London Paddington and Airport Junction, which allows Heathrow Express and Heathrow Connect trains to operate. This electrification will be extended as far as as part of the Crossrail project.