This is mentioned in the video published by the White House.
According to Biden, he could not arrive in Ukraine by plane because there were concerns in the US that he might be shot down. That’s why they decided to use a train instead.
However, the US Secret Service had concerns about this. They believed that “they (the Russians - ed.) will know that we are here,” and “someone might do something to sabotage the train.”
“When I got off the train, we were walking through an area when suddenly the sirens went off. Zelensky and I were determined not to stop and keep moving forward because those sirens meant we were about to be attacked,” the US president recounted.
The video also features remarks from Ukrainian President Volodymyr Zelensky, who stated that he and Biden had to go to shelter “very quickly.”
“And it is very important that Biden saw this with his own eyes, not through the eyes of his intelligence. That’s why I believe he could truly share this with his people, because he was here,” Zelensky said.
He also added that Biden's visit was very significant for “boosting the morale” of Ukrainians. This was the first time in history that a US president visited a war-torn country not controlled by the American military, according to the Ukrainian leader.
On the morning of February 20, 2023, with an unannounced visit to Kyiv, President Joe Biden of the USA arrived. This was his first visit since the onset of the full-scale war and since the beginning of his presidential term in 2021.
In Kyiv, Biden met with President Volodymyr Zelensky and announced a new package of military aid for Ukraine. The visit took place on the Day of Commemoration of the Heroes of the Heavenly Hundred, as well as on the eve of the anniversary of Russia's full-scale invasion.
Journalists reported that Biden's visit was kept secret until the last moment. It was planned for months by several top aides to the American president.