Changi Airport's inter-terminal Skytrain system will be expanded by 2019, with additional train cars and upgraded infrastructure.
A contract was awarded in March to Mitsubishi Corporation and Mitsubishi Heavy Industries to upgrade the driverless transit system, The Straits Times has learnt.
Six new train cars will be added to the system to cope with the "expected increase in ridership" as passenger traffic grows, said a spokesman for Changi Airport Group (CAG).
The Skytrain from Terminal 3 to Terminal 2 - which serves both public and transit areas - will be upgraded from a two-cabin train to a three-cabin one, CAG added.
Also, the current single-cabin Skytrain serving transit passengers at Terminal 3's "A" boarding gates will get an additional cabin, CAG said.
The project also involves "upgrades and additions to signalling and communication systems and related station facilities" for the 6.4km system, Mitsubishi said in a press statement.
While CAG did not reveal the cost of the project, Japanese newspapers said it is "some billions of yen".
Since it opened in 1990 to connect Terminals 1 and 2, the Skytrain has gone through a series of upgrades, including a refurbishment from 2002 to support the opening of Terminal 3 in 2007.
The current Skytrain uses Mitsubishi's Crystal Mover train system - which is also employed at airports in Dubai and Hong Kong. The new train cars purchased by CAG will be similar to those in its current fleet.
The upgrades will expand the Skytrain's capacity to cope with an expected increase in passenger traffic after Jewel Changi Airport is completed in 2018, Mitsubishi said.
Jewel Changi Airport is a mixed-use complex with retail outlets and airport services, in front of Terminal 1.
Changi handled a record 55.4 million passengers last year, a 2.5 per cent increase from 2014.
The free transit system is not only used by travellers, but has also become popular with families.
Mr Abraham A., 33, an insurance executive who has a daughter, four, and son, two, said: "It can get quite crowded on the weekends. My kids don't usually take the MRT, so getting on the Skytrain is a real treat.
"Having more train carriages will be good."
Correction note: An earlier version of the story referred to Jewel Changi Airport development as Project Jewel. This has been corrected.