South Korea says it would be on board with the plan as a high-speed rail network could unlock trade and tourism in the peninsula by connecting it to Russia, China and beyond.
Japan is planning to bolster its systems for monitoring air activity by North Korea, which has performed ballistic missile tests over Japan in the past.
Singapore's small community of N. Koreans has kept a low profile in recent years as diplomatic pressure on their home country made working abroad increasingly difficult.
US ambassador to the UN Nikki Haley told reporters the United States was hearing reports that North Korea might be preparing to fire another missile.
Prime Minister Prayuth Chan-ocha denies US claims that North Korean companies are doing business with Thailand.