Did you notice your mobile phone coverage on the train ride into work on Thursday morning was a little better?

If you were travelling on the eastern suburbs line in Sydney, that would be why. It was the last line to join the underground transport network's mobile coverage upgrade that begun last year.

Rolling reception: Optus has switched on mobile phone coverage for Sydney's Redfern to Bondi Junction tunnels.

Commuters can now make calls, send texts and browse the web while hurtling through tunnels after Optus completed the last phase of a multimillion-dollar project to provide coverage in one of Sydney's most annoying blackspots.

The last phase of the upgrade involved the rollout of 18 kilometres of "leaky" cabling.