Complex sentence: Crossrail will have platform screen doors at most of its underground stations; except the stations at Heathrow Airport, which will also be served by Heathrow Express trains, which have a different door layout.
Split into simpler sentences: Crossrail will have platform screen doors at most of its underground stations, except for those at Heathrow Airport. These stations will also be served by Heathrow Express trains, which have a different door layout.