PROPERTY tycoon Mr Li Ka-shing has offered to build a $15 billion light rail between Cheung Sha Wan, in central Kowloon, and the rapidly-developing Ma On Shan. The railway, which would take four years to build if approved, may also connect to the proposed airport railway. A project, now circulating among government departments, has been spearheaded by Mr Li's eldest son, Mr Victor Li Tzar-kuoi, the deputy managing director of Cheung Kong (Holdings). This is the second attempt by Cheung Kong to get the idea off the ground. Its proposed $3 billion, elevated light rail system between Tsim Sha Tsui and Hung Hom was rejected by the Government last month. The new proposal involves a light rail line running from Ma On Shan to Tai Wai, which is an existing Kowloon-Canton Railway station, and through Kowloon to Cheung Sha Wan, where the MTR will build a station on the future Lantau Line of the airport railway. A large part of the railway line will be elevated. Population growth along the route is seen by Cheung Kong as sufficient justification for the investment. The 310-hectare new town of Ma On Shan is expected to have a population of about 200,000 by the year 2000, with 70 per cent being housed on public estates. Cheung Kong recently swapped its Tsing Yi oil depot site with the Government for a 27,500-square metre residential site at Ma On Shan. In its submission, the company has also asked the Government to provide several plots of land along the railway line for property development as a form of assistance in building the project. The feasibility study was submitted to the Government last month and is being studied by the Transport Branch, the Transport Department and other relevant technical departments such as the Planning Department. They will study the project with other comments to be submitted during a three-month consultation on a Railway Development Study, which will end in June this year. The Government started an in-depth study into railway development in 1979 and identified nearly 100 proposals. Six of them were incorporated in the study for consultation - including the MTR Tseung Kwan O extension and the second heavy rail from Shamshuipo to Lowu. The engineering representative in the Legislative Council, Mr Samuel Wong Ping-wai, said a railway line running from Ma On Shan to Cheung Sha Wan would not be the most effective means of serving the population there. ''Buses would be better,'' he said. Mr Wong also said there might be difficulty for the Government to grant land to private consortiums in exchange for infrastructural projects, as it would need approval from the Chinese side. The Government earlier studied the viability of either a railway line from Ma On Shan via Tai Wai to the West Kowloon Reclamation, or from Ma On Shan via Tai Wai to Cheung Sha Wan before Cheung Kong's plan. The Ma On Shan-West Kowloon Reclamation plan was estimated by the Government to cost $25.7 billion, more expensive than the Cheung Kong proposal. However, both lines were ranked as low priority when compared to the Tseung Kwan O MTR extension and the Shamshuipo-Lowu heavy rail. ''They have low priority since the level of patronage is not high. If the rate of return is not good enough, the lines would then have a problem of attracting sufficient passengers and then their fares would be raised further lowering the level of patronage,'' he said.