Complex sentence: Crossrail will have Platform Edge Doors at most of its underground stations, the exceptions are the Heathrow Airport stations as these will also be served by Heathrow Express whose trains have a different door layout.
Split into simpler sentences: Crossrail will have Platform Edge Doors at most of its underground stations except for at the Heathrow Airport stations. These stations will also be served by Heathrow Express trains, which have a different door layout.