I can't find a date in the article itself, but the source code has a date of October 24, 2017.
Mombasa-Nairobi Standard Gauge Railway Project
Kenya Railways Corporation is developing a new standard gauge railway (SGR) line for passengers and cargo transportation between Mombasa, the largest port in East Africa, and Nairobi, the capital city of Kenya. ... The new
railway line constitutes the first phase of the SGR project that aims to connect Kenya, Uganda, Rwanda and South Sudan.
The Mombasa-Nairobi SGR is the biggest infrastructure project in Kenya since independence. It will shorten the passenger travel time from Mombasa to Nairobi from more than ten hours to a little more than four hours. Freight trains will complete the journey in less than eight hours.
Construction of the 609km-long line began in October 2013 and is scheduled to be completed by December 2017. At least 60 new jobs a kilometre of track or approximately 30,000 jobs are expected to be created during the construction. ... The Mombasa-Nairobi phase of the project is estimated to cost KES327bn ($3.8bn). China Exim Bank will provide 90% of the financing while the remaining 10% will be contributed by the Kenyan Government.
....
Contractors involved
China Road and Bridge Corporation, a subsidiary of China Communications Construction Co., was contracted to build the project according to Chinese railway design standards.
Wikipedia says it has been open for a year and a half already:
MombasaNairobi Standard Gauge Railway
The MombasaNairobi Standard Gauge Railway is a standard-gauge railway (SGR) in Kenya that connects the large Indian Ocean city of Mombasa with Nairobi, the country's capital and largest city. This SGR runs parallel to the defunct, narrow-gauge Uganda Railway that was completed in 1901 under British colonial rule. The East African Railway Master Plan provides for the MombasaNairobi SGR to link with other SGRs being built in the East African Community.
At a cost of US$3.6 billion, the SGR is Kenya's most expensive infrastructure project since independence. The prime contractor was the China Road and Bridge Corporation, which hired 25,000 Kenyans to work on the railway. Phase 2A of this project was launched officially in October 2016.
The first fare-paying passengers boarded the "Madaraka Express" on Madaraka Day (1 June 2017), the 54th anniversary of Kenya's attainment of self-rule from Great Britain. Commercial freight services began on 1 January 2018. Passenger uptake has exceeded expectations, with the train carrying 2 million riders in the first 17 months of operation.