Being able to use your phone (or mobile broadband) on the train is great for productivity, but it's a minor nuisance when your train plunges into the tunnel and the signal disappears. That's going to become slightly less common in Sydney, with Optus enhancing coverage on the stretch of rail between Chatswood and Central to include in-tunnel coverage.
Picture by Nick
Optus has been testing this option for a while, but it officially switched it on last night. Right now, the coverage doesn't cover the other CBD City Circle tunnels that are underground (St James, Museum and the connections to Circular Quay), but Optus says those will be introduced eventually. Telstra has also been testing improved network coverage throughout the City Loop in Sydney, but that's not yet officially rolled out either.
Update: Telstra has just announced that its Next G network (including HSPA+) between Central and North Sydney stations is now up and running as well. Unlike Optus, it doesn't extend all the way to Chatswood, but Telstra says 4G coverage will be integrated soon as part the extension.
The newer tunnels in the Sydney network (on the airport link and between Chatswood and Epping) have always offered mobile coverage. No such luck in the other tunnels I know of in Australian cities, though with the exception of Melbourne's City Loop, the tunnels in Brisbane and Perth are relatively short in length.