Waypoint Echo is a location in the Commonwealth in 2287. It only becomes active if the Sole Survivor is allied with the Brotherhood of Steel and helps them reactivate Liberty Prime during the quest Liberty Reprimed.
A small Brotherhood of Steel outpost right at the Edge of the Glowing Sea, consisting of a mobile communications array, a semicircle of sandbag barriers and a lone machine gun turret. It plays a minor role as the Brotherhood's staging area for their search for Liberty Prime's Mark 28 nuclear bombs in the Glowing Sea, but serves no purpose otherwise.
During the quest Liberty Reprimed, the waypoint is garrisoned by Scribe Haylen and two more Brotherhood soldiers. The latter remain there afterwards and respawn upon death every couple of in-game days even if the Brotherhood has been destroyed during one of the final main quests.
The respawning, power-armored Brotherhood soldier provides a limitless and easily accessible supply of T-60 power armor pieces. If one manages to pickpocket or destroy their fusion core (easy to do with the Penetrator perk), their power armor frame can be salvaged as well every couple of in-game days. Note that entering such a frame will be considered stealing.
Be careful when fast-travelling to the waypoint as an enemy of the Brotherhood. Doing so will place the player character squarely in arm's reach of a live turret and two heavily armed sentries, one of them clad in power armor. Although they will usually be facing away from the Sole Survivor, almost any move will immediately draw their attention and incite them to attack.
Waypoint Echo only appears in Fallout 4.