President Uhuru Kenyatta is expected to attend the launch ceremony alongside Chinese Ambassador to Kenya, Wu Peng.
The extension connects the capital, Nairobi, to Naivasha town, where the Kenyan government expects to build a dry port for inland cargo services.
The 120 kilometer railway line has been built by the China Communications and Construction Company (CCCC), and is mainly funded by the Chinese government.
The SGR is billed as the biggest project in Kenya since independence, and is part of an ambitious government plan to connect the coastal city of Mombasa to the Western city of Kisumu.
The Mombasa-Nairobi phase of the project was built at cost of $3.8bn, with China Exim Bank providing 90% of the financing while the remaining 10% was contributed by the Kenyan Government.
Ahead of Wednesday’s launch, CCCC has been conducting test runs over the past two months with a focus on safety and service delivery.