Sydney Australia has had this for several years, including through it's various tunnels. Whilst the majority of the network is above ground, the inner-city is underground. The telcos here spent a lot of money/time in getting 100% coverage through the tunnels in ~2011 though, which definitely enabled something like this.
Glad to hear that cell signal is finally reaching across the subway network - when I was last in NYC it drove me nuts not being able to just browse on my phone whilst waiting for a train.