The railway line is eventually expected to connect Uganda, Rwanda, Burundi, Democratic Republic of Congo, South Sudan and Ethiopia to Mombasa.
Kenya inaugurated a $3.2 billion railway funded by China linking the capital Nairobi to the port of Mombasa on Wednesday.
It is the country's biggest infrastructure project since independence more than 50 years ago.
The railway is part of China's "One Belt, One Road" initiative, a multi-billion dollar series of infrastructure projects upgrading land and maritime trade routes between China and Europe, Asia and Africa.
The line is eventually expected to connect Uganda, Rwanda, Burundi, Democratic Republic of Congo, South Sudan and Ethiopia to Mombasa so the Indian Ocean port can act as a gateway to East Africa for trade with China and other nations.
The railway was finished well ahead of schedule by China Road and Bridges Corporation and launched at the peak of campaigning ahead of elections on Aug. 8 in which President Uhuru Kenyatta is seeking a second term.
The express will slash the time for the 472 kilometres (293 miles) journey to about four hours from 12 on the line built by British colonialists more than a century ago that stretched from Mombasa to the Ugandan capital and was nicknamed the "Lunatic Express".
The old line was criticised as a monumental folly and waste of British taxpayers money but it became a strategic and economic lifeline for the colonial power. Thousands of workers from British India died during its construction, including a number dragged from their tents by lions.
The new railway was Kenyatta's pet project and it was a key pledge in his 2013 election campaign. He christened the new line the "Madaraka Express", named after the June 1 holiday that marks the day in 1964 Kenya won self-governance from Britain ahead of full independence.
The express gives businesses and passengers a cheaper and safer alternative to the notoriously dangerous trip along the sometimes pot-holed single-lane highway between the two cities that is often clogged with cargo trucks.
"We celebrate laying one of the key cornerstones to Kenya's transformation to an industrialised, prosperous, middle-income country," Kenyatta said ahead of the inaugural trip from Mombasa on Wednesday.
Talk of China
The importance of the line to Kenya was underlined by the government's response to the arrest of four men stealing guard rails and cables. It said those guilty of economic sabotage could be hanged.
Aboard the train, talk of China, which has been vying with Western and Asian nations for influence in sub-Saharan Africa, was on the lips of passengers and workers alike.
In the flag-draped carriages, many of the waitresses in black dresses and blue-suited waiters were eager to discuss their Chinese-language studies.
"Its a tough language to learn," waiter Zachariah Ropia said as he moved down the sleek white carriage striped with orange, amber and yellow. "I'm pressing on because of my job."
"A country cannot progress without good infrastructure," said businessman Michael Kariuki, sitting in a first-class carriage. "Anybody who cannot see what China has done for Kenya must be blind ... China is the way to go."