A view from Singapore of the border crossing into the Malaysian southern city of Johor Bahru.

Singapore and Malaysia may find their relationship under pressure if the latter's new government withdraws from a rail project that's become emblematic of improved bilateral ties.

Uncertainty clouds the future of a 2016 deal to construct a high-speed rail network between Kuala Lumpur and Singapore after Malaysian Prime Minister Mahathir Mohamad said this week that all foreign infrastructure programs will be reviewed. His administration will be assessing existing initiatives on feasibility of as well as the sum of borrowed money.

Scrapping the 350km rail network, which is expected to slash travel time between the two cities to 90 minutes from a four to five-hour car journey, could reintroduce tensions into Malaysia's interactions with its wealthy, next-door neighbor.

For decades, relations between the two were uneasy after Singapore separated from Malaysia to become an independent nation in 1965. The two now enjoy a warm alliance — despite occasional hiccups over issues such as territory and water — that the Kuala Lumpur-Singapore high speed rail project is meant to represent.

"The rail is a very important project symbolizing the mending of ties between Singapore and Malaysia in the 2010s," said Chan Xin Ying, a Malaysia-focused research analyst at Nanyang Technological University.