The government's megaproject investment is likely to use domestic lending sources in the initial phase with foreign lenders joining in the later stage, says the Asian Development Bank (ADB).
Based on discussions with the Finance Ministry, Yasushi Negishi, the country director for the ADB's Thailand Resident Mission, said he understood that the government wanted to raise initial funding for the projects from the domestic market. But external sources will be considered in the later stages.
"The time will come for the ADB to provide financial assistance," he said.
"At the moment, we are talking about financing Bangkok's urban mass transit such as the underground and monorail systems. I think those are the projects the ADB is pretty keen to support."
The bank is also interested in the government's negotiations with China and Japan concerning the construction of the dual-track railway, said Mr Negishi.
Thailand needs to invest in railway and mass-transit systems in order to upgrade the country's infrastructure and stimulate the economy, he said, adding that Thailand has not invested much in large-scale infrastructure for decades because of the political changes.
The government earlier declared 2016 the year of investment after consumption and exports had slowed.
In December, the cabinet approved action plans for 20 infrastructure projects worth a combined 1.79 trillion baht, part of the strategy to kick-start an ambitious development plan from 2016-22.
The cabinet also approved the launch of an infrastructure fund to raise cash from the public for the construction of state projects.
The Thailand Future Fund will mobilise up to 100 billion baht and will be listed on the Stock Exchange of Thailand. The Finance Ministry and the Vayupak Fund will initially put 10 billion baht as seed money for the fund to bolster investors' confidence and to seek more income.
Mr Negishi said the ADB would charge an annual interest rate of 0.5% based on the US dollar London Interbank Offered Rate for loans provided to middle-income countries such as Thailand.
"We believe this amount is attractive for middle-income countries because if Thailand borrows from the capital market or commercial banks in terms of US dollar financing, [then] it has to pay the base rate of 1.3-1.7%," he said.
He acknowledged the US Federal Reserve's interest rate normalisation would affect the short-term interest rate offered by the ADB since the bank offers loans in US dollar terms.