Moscow's Metro is one of the most-used underground railways in the world, carrying about 5.5 million passengers a day.

BBC: Moscow Metro hit by deadly suicide bombings

Despite its relatively central location just northwest of the Kremlin, the only way to get to the complex is either by infrequent metro service on a newly built line, or via narrow off ramps from Moscow's busiest ring road that often are clogged by construction vehicles.

WSJ: Moscow Business Complex Sees Rebirth

The Christmas Eve service in Moscow's Christ the Saviour began at 2200 local time, and the city's metro laid on special trains to allow the many worshippers to return home late.

BBC: Orthodox Christians celebrate Christmas