Police revise the earlier death toll, from 19.
The train, which was carrying 78 passengers and five crew, was part of a newly expanded faster rail service along the route linking Seattle and Portland, Oregon - featuring new locomotives.
A No. 872 bus lost control and toppled on Tai Po Road Saturday evening, local radio RTHK reported.
Police said at least 18 people, including 15 men, had been confirmed dead on the scene.
Dozens were injured in the wreck, police spokeswoman said. Though there is no confirmation about the cause of the accident, reports said the vehicle was speeding. And then it was like the tyres slipped and the bus turned. "People fell on one another and got tossed from side to side".
Photos and footage from the scene showed the roof of the bus had been torn off.
The injured passenger told the paper: "It was much faster than I normally felt in a bus".
Hong Kong's public transport is usually safe, and deadly accidents of this scale are relatively infrequent.
In 2008, 18 people were killed in another bus crash.
It is understood 40 people were injured.
The safety of Hong Kong's waterways has also remained under scrutiny since 39 people were killed when a high-speed ferry and a pleasure boat crashed in 2012.
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