A metro: frequent underground train found in built-up areas with high populations.
This emoji is generally displayed as a forward-facing train, with some platforms also displaying a tunnel as a circle surrounding the train. Apple's artwork shows the characters IL2 which may be a reference to one of the more locked-down buildings at Apple's Infinite Loop Headquarters during iPhone development (IL2 = Infinite Loop 2).
Metro was approved as part of Unicode 6.0 in 2010 and added to Emoji 1.0 in 2015.
U+1F687
? Metro