Two factors are crucial if the planned high-speed rail link between Singapore and Kuala Lumpur is to be completed by 2020, said experts yesterday.
There must be strong political will and most decisions will need to be finalised by next year.
Even then, it will be touch-and-go, they added, as many other key issues need to be resolved.
These include the final location of the terminal in Singapore, which has identified three possible sites; the alignment of the rail line and whether it will run on ground level, be elevated or underground; and financing of the project.
But if the two governments can define all the key parameters by end-2015, Mr Barry Howe, vice-president (marketing and strategy) at rail systems supplier Alstom Transport Asia-Pacific, believes it would be possible to complete the project by 2020.
He was commenting on the 2020 deadline Prime Minister Lee Hsien Loong and his Malaysian counterpart Najib Razak had set for the 320km to 340km rail project, which will provide a 90-minute commute between Singapore and KL.
Meeting the deadline hinges on the rail being at ground level, which would be "fairly simple" to build, said Mr Joshua Ong, vice-president of engineering consultancy Jurong Consultants.
"You just need to get the ground compacted and lay the tracks," he noted.
Going underground, however, will take "twice as long or more", he said.
For instance, construction of the proposed Tokyo-Nagoya maglev system, which is largely underground, is expected to take 13 years.
The maglev will cut travel time between the two cities - their distance is about that between KL and Singapore - from 90 minutes on the Shinkansen bullet train to 40 minutes.
Mr Rajan Krishnan, chief executive of engineering firm KTC Group and former head of rail projects at the Land Transport Authority, envisages a large part of the line in Malaysia to be at ground level as going underground most of the way "will not be economically viable".
He foresees land acquisition to be the main issue. "If land had already been earmarked for such a project, then it will be easier."
Looking at how China built its high-speed rail network, Mr Ong said: "If there's a will, there's a way."
China began planning its high-speed rail system in the early 1990s. Today, its network is the world's longest, spanning more than 10,000km.
In contrast, it took Taiwan nearly 30 years to roll out a 345km high-speed rail link between Taipei and Kaoshiung. Construction took about seven years, but the planning took nearly 20.
While an underground system will be up to five times costlier to build than a ground-level line, it may be more cost-efficient in the long run, said industry observers.
For instance, it will be safer as intrusions by humans and animals can be eliminated. Also, speed will not be hampered by wet weather.
But Mr Krishnan also noted that intrusions can be prevented "by erecting high fences, which are used widely in Europe".
As for the cost of the project, an estimation is difficult as there are too many unknowns, said industry players.
But Mr Colin Stewart, global rail leader at international engineering consultancy Arup, had told The Straits Times in an interview last year that it should range from £15 million (S$31.4 million) to £40 million per km, depending on the complexity of construction.
If so, the KL-Singapore project could cost as much as $30 billion, excluding land costs.
Cost is also dependent on the system. The Tokyo-Nagoya system, which uses the latest magnetic-levitation trains capable of 500kmh speeds, is estimated to cost $110 billion, or $171 million per km.