Tracks in the undeveloped area are used for little more than parking idle trains and the MTR says it is looking at alternative uses for the space.

The situation raises new questions about the MTR Corp's estimates for its multibillion-dollar projects, many of which are still well below the forecasts, as work on the most expensive yet - the HK$65.2 billion express line to Guangzhou - is about to begin.

An economist said the MTR Corp appeared to have a tendency to roll out over-optimistic projections to paint a rosy picture for its projects. 'Because of the continuously changing economic environment it is difficult to say whether it over-stated the passenger figures for the Airport Express, but it is inevitable that the company would provide favourable figures,' Timothy Hau Doe-kwong, associate professor of economics of the University of Hong Kong, said. 'There is a possibility that the high-speed rail will turn up as a white elephant if the MTR Corp is only quoting favourable figures, as it did with the Airport Express.'

The 24-minute airport rail service, which cost HK$35 billion and opened in July 1998, carries about 29,000 passengers a day.

The MTR Corp originally estimated 39,000 passengers a day in the first year, rising to 54,000 in 2006 and 75,000 in 2011. But passengers have shown a preference for cheaper, more convenient buses, and the railway's share of airport visitors has fallen from 50 per cent in 1998 to about a third last year.

To meet the demand predicted at the planning stage, two platforms were built at Hong Kong station from which a train of 10 cars could leave every eight minutes. Between 1999 and 2003 however, the daily passenger load fell by more than 30 per cent, from 28,500 to 18,800. It rose gradually to about 29,000 a day last year. Now, a seven-car train leaves the sole active platform every 12 minutes.

Hau said the variety of available transport to the airport contributed to the loss. 'Many models show that passengers prefer point-to-point transportation to the airport. Airport buses and minibuses offer point-to-point transportation, which is more flexible and much cheaper,' he said.

The Airport Express from Hong Kong station to Airport station costs HK$100 for a single trip, while bus route A11 from Hong Kong Island costs HK$40. Hau said it was difficult for the MTR Corp to offer a more attractive price because of the high cost of building the express.

Leung Kong-yui, former transport advisory committee member and former president of the Chartered Institute of Logistics and Transport, expects it will take another 20 to 30 years to reach the originally estimated capacity.

MTR Corp senior manager Maggie So Man-kit said the airport's passenger numbers were expected to increase to more than 70 million by 2020 and beyond so there would be a need to increase the service from the current five trains an hour to eight trains an hour.

'Once the train frequency increases to seven trains per hour or above, the second platform and additional tunnels will be required,' So said. She said some alternative uses of the reserved platform had been explored, but nothing materialised due to land-use restrictions and high renovation cost.

Meanwhile, other rail lines in the city are also struggling to meet their expectations. Predicted daily passenger load on the HK$10 billion Lok Ma Chau spur line, which opened in 2007, was 55,000 and 70,000 in its first year. But the line from Sheung Shui and Lok Ma Chau is carrying only about 30,000. The HK$10 billion Ma On Shan line was predicted to serve 190,000 passengers a day in the first year of operation in 2005, but carries only 90,000.

These figures have prompted questions on whether the 140,000 square metre terminus to be built under West Kowloon for the Guangzhou-Shenzhen-Hong Kong high speed link, is cost-effective.

Without revealing the passenger projections, the Transport Bureau estimated that the HK$65.2 billion high-speed rail line would carry 99,000 passengers when it begins service in 2016. There will be 15 platforms at its terminus in West Kowloon.

Albert Lai Kwong-tak, chairman of the think tank Professional Commons, said the government had overestimated the number of passengers using the Airport Express, and had given the MTR Corp more land than was necessary. He said that what could be done now was to make the best use of the space.

Professional surveyor Raymond Chan Yuk-ming said that the rental value of underground space in Central ranged from HK$20 to HK$300 per square foot depending on its accessibility.

The presence of pockets of idle land raised the eyebrows of Christine Loh Kung-wai, chairwoman of the Society for Protection of the Harbour.

She questioned whether the government's claims for 'minimum reclamation' had been genuine.

'It is [often] impossible for a member of the public to tell from the drawings whether the land requirements are necessary. But in our last legal battle with the government, [we saw] that the government managed to cut back from reclamation, even its plan was the 'minimum',' she said.