Unlike other carriers, T-Mobile's online coverage map has never shown areas of LTE coverage, only visually mapping out 2G coverage areas.
T-Mobile instead has periodically provided a list of cities with LTE, making it extremely difficult for travelers to have any broader sense of where T-Mobile might work best for them.
The only way to check T-Mobile LTE coverage before now was to search for specific addresses one at a time, a very time consuming process.
Fortunately - it looks as if things are about to change.
We are looking forward to seeing this new mapping tool, and will do our best to incorporate improved T-Mobile maps into the next update of our app 'Coverage?'.
UPDATE: T-Mobile did indeed update their website with new more detailed maps, as promised. But, just like the old maps, the T-Mobile interactive map viewer lumps 3G, 4G (HSPA+), and LTE coverage into the same layer - making no visual distinction between them. To actually see what kind of coverage is in a given area, you can click on the map to bring up a popup with more details.
It is frustrating for travelers that T-Mobile provides no easy way to see where LTE will be along a give route.