"The water reached my chest," a survivor wrote on social media. "I was really scared, but the most terrifying thing was not the water, but the diminishing air supply in the carriage."
From the evening of Saturday until late Tuesday, 617.1 millimetres (mm) of rain fell in Zhengzhou, about 650 km southwest of Beijing.
The amount of rainfall in Zhengzhou witnessed over the three days was one seen only "once in a thousand years", local media cited meteorologists as saying.
Flood prevention difficult
The lives of millions of people in Henan, a province with a population of around 100 million, have been upended in an unusually active rainy season that has led to the rapid rise of a number of rivers in the vast Yellow River basin.
Many train services across Henan, a major logistics hub in central China, have been suspended. Many highways have also been closed and flights delayed or cancelled.
Roads in a dozen cities have been severely flooded.
"Flood prevention efforts have become very difficult," president Xi Jinping said on Wednesday, addressing the situation in a statement broadcast by state television.
Dozens of reservoirs and dams also breached warning levels.
Local authorities said the rainfall had caused a 20-metre breach in the Yihetan dam in Luoyang city west of Zhengzhou, and that the dam "could collapse at any time".
In Zhengzhou, the local flood control headquarters said the city's Guojiazui reservoir had been breached but there was no dam failure yet.
About 100,000 people in the city have been evacuated to safe zones.