T-Mobile officially launched its low-band, 700MHz LTE network in New York, Cincinnatti, and Seattle today. This makes 246 metro areas where the carrier has turned on the 700MHz network, and obviously we haven't reported on all of them. But two of those markets, New York and Cinci, are especially big deals.

New York is the nation's biggest city, and T-Mobile's biggest single market. While 700MHz spectrum in most of the country means better coverage of exurbs and rural areas, in New York it means better in-building penetration, T-Mobile engineering director Salim Kouidri said.

The rollout started back in July, when New Jersey's public broadcaster cleared TV channel 51—that's T-Mobile's 700MHz band—and made room for T-Mobile to start building out. Wireless spotters were seeing sites active this fall, but it took until now to get elements like site-to-site handoff right, Kouidri said.

The new spectrum will also improve speeds when used with a carrier aggregation-capable phone such as the iPhone 6s or Samsung Galaxy Note 5, he said.

"We're looking at 30-45 percent speed augmentation," he said.

Eventually, the 700MHz spectrum will help T-Mobile close the gap between subway stations, too. Kouidri said that broadcasting 700MHz from platforms can reach between local stations on the Lexington Avenue line—although that will need cooperation from T-Mobile's buildout partner Transit Wireless, which will take some more time.

"A lot of the platforms are connections through direct line of sight, unless you take an express, and the cutoff point is exactly where 700MHz can patch the two ends. But that's a long-term plan," he said.

New York City had pretty decent T-Mobile coverage before, though. The carrier's problem in the New York metro area is more in the northern and northwestern surbubs—counties like Rockland, Orange, Putnam, and Duchess. T-Mobile's Band 12 spectrum covers everything up to the Catskills, and according to this map, there have already been a few sightings of coverage in the area. Kouidri said T-Mobile's buildout north of the city was sometimes slowed down by having to go to each individual town for permits, but that the carrier was working on it.

"This is just a start," Kouidri said. "We're beefing up the city, and then going out."

In Cincinnati, meanwhile, T-Mobile historically had a small allotment of AWS spectrum, resulting in a very constrained LTE network. That was boosted by the use of 1900MHz spectrum in May and now has been supplemented with the 700MHz spectrum. That will make the carrier considerably more competitive in southern Ohio.