Gatwick and Heathrow airports have both seen the completion of rail links in recent weeks.
At Heathrow, the rail link between T5C and the main building at Terminal 5 has been finished.
T5C is T5's second satellite building, after T5B, and is due to open next summer. It will have an additional 12 aircraft stands, eight of which will be able to accommodate the A380.
The underground connection is a vital step on the road to opening the new satellite, which will take the two-year old terminal to its full capability.
The light rail system is made up of two trains, each with three cars large enough to carry around 60 passengers.
Each 16-tonne car had to be lowered through a hatch on the airfield, in a three-night long operation. Steven Morgan, BAA capital director, said the delivery of the cars at the busy airport had been a major challenge.
"The fact that this has been undertaken so safely and successfully on the world's busiest international
airfield, with no impact on daily operation of the TTS, is a credit to all involved," said Morgan.
"The TTS is an essential service for passengers using Terminal 5 and it will be even more important when T5C opens."
Gatwick airport has also completed work on its 20-year-old rail service, which links the North and South terminals.
The inter-terminal shuttle reopened ahead of schedule after a 10-month closure, during which passengers had to travel between the two terminals by bus.
The £43 million project saw the installation of a new shuttle system and new shuttle cars, as well as refurbishment of the track and the two terminal stations.
The airport, which became independent from BAA in December after being bought by London City owners Global Infrastructure Partners, has also introduced sensory technology to improve timings.
The technology counts the number of passengers waiting at the station, triggering the operation of the shuttle.
James van Hoften said, "The original inter-terminal shuttle made the equivalent of five return trips to the moon during its 20-year lifetime - something I've done a couple of times during mine!
"The new facilities are excellent but it's one small step in our plan to make Gatwick a great airport that passengers love to use."
Gatwick is undergoing a £1 billion investment programme over the next three years, which will include improvements to the South Terminal departure lounge, security areas, immigration halls and baggage systems.
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