If you travel on the subway in the London Underground, you'll hear various automated recordings of instructions and announcements for passengers. But at one stop, one instruction stands out from the rest—a unique voice warning people to "Mind the gap."

It's not actually new. The same voice issued the same warning for decades, but was replaced in 2012 when the Underground installed a new digital system.

Weeks later, though, it was back. Why? Because kind Underground workers wanted to help a grieving widow who missed hearing her late husband's voice.