Telstra, Optus and Vodafone have completed phase one of a network expansion that will see mobile coverage brought to commuters in Sydney's City Circle and Eastern Suburbs rail tunnels.
The telcos have switched on network equipment that provides coverage on the North Shore line between Central Station and North Sydney.
Further expansion of coverage in tunnels — including the City Circle loop — is expected later this year.
A spokeswoman for Vodafone Hutchison Australia said that equipment would also be switched on further up the North Shore line.
"The railway tunnel program will initially benefit Vodafone customers travelling on the North Shore line, with equipment being switched on in the Wynyard North Tunnel, Central to Town Hall and Town Hall to Wynyard by the end of July, and in the North Sydney and Chatswood tunnels in the coming months," she said.
"We are installing our own 3G and 2G Base stations and co-locating them onto an Optus Active Distributed Antenna System which has been installed in each of the tunnels."
Telstra said it had rolled out dual-channel HSPA+ technology but would upgrade to LTE "soon". Optus has also rolled out 3G technology.
Telstra's director of network and commercial planning, Anthony Goonan, said the project was "many years" in the making and had been given the go-ahead late last year by NSW Transport Minister Gladys Berejiklian.
The state's Premier Barry O'Farrell said on Twitter that it was "great to see [Berejiklian] improving wifi coverage on CityRail services".