T-Mobile 700MHz LTE signal (Band 12) spotted in Detroit
According to the Service Mode, the T-Mobile pipeline on LTE Band 12 has a 5MHz x 5MHz bandwidth. This past Summer, T-Mobile CFO Braxton Carter said that eventually, all of the metro areas where T-Mobile operates will offer bandwidth of 20MHz x 20MHz. Larger bandwidth allows more traffic to use the carrier's pipeline, without suffering the network congestion that usually slows down data speeds.
According to crowdsourced app Open Signal, T-Mobile offers the fastest LTE service in the country. That report confirmed earlier data from Speedtest.com that also called T-Mobile's 4G LTE service the fastest in the country.