Metro tiles seem to be like Toile de Jouy, the more the merrier, in Montreal we call them metro tiles, because we use the metro not the subway, in London England they call their metro the Tube.
Via http://down2thewire.me/
Subway tiles were used in the New York subway when it opened in 1904, the walls were covered with subway tiles and it somehow became quite popular in subways and in homes around the world.
Via Absolutely Beautiful Things}

Top to bottom metro tiles behind your bathroom vanity using a gray grout for a light contrast with the white tiles.
Via Elle Decor

The metro tiles can come flat or beveled, white or multicolored, porcelain or glass.

For a more dramatic effect in your kitchen, use a black grout, very Coco Chanel, don’t you think so. By the way, Coco also said that ”La mode se démode” meaning that trend go out of style, but they are certain things that never go out of style like Chanel and the ”subway tiles”.
Via http://wirednewyork.com

New York subway with its white subway tiles.
Often mosaic patterns were inserted within the subway tiles, bu this is a more expensive venture.

New York subway tiles herringbone style, you can install your metro tiles at a 45 degree angle, but it is a bit more costly, allow for at least 15% more tiles.