Moscow's metro said on Friday it was carrying out emergency checks of its system after a young couple made their way into a tunnel and had sex on the rails.
Moscow has a subculture of enthusiasts who explore its maze of tunnels including the metro system, called "diggers", and a young couple this week published photos of themselves romping naked on the rails.
Their feat came to light only after the couple posted their photographs on social networking sites, prompting questions about the ageing metro's security.
The metro management announced spot checks of the security on the whole system to see if it was possible to get illegal access to the underground zones, a spokesman said.
"In particular, all the ventilation shafts will be checked to see if they can be used to access the underground system," the spokesman said.
The couple picked a service tunnel that is only used to divert trains in an emergency, not one along which metro trains rumble up to every half minute at peak times, the metro spokesman added.
"If such an incident had taken place on a constantly operating stretch of track, the wrongdoers would have been discovered and caught immediately, as soon as they entered the tunnel."
Police said Thursday that they were carrying out a check into the incident and trying to establish the identity of the couple.
The male digger, who only gave his name as Andrei R., told Life News website that on Tuesday he and his girlfriend entered a service tunnel on the green metro line, which runs close to the Kremlin.
"We were definitely not the first," he said.
Last year, a couple's attempt to have sex on railway tracks in southern Ukraine ended in tragedy when they got carried away and failed to notice an oncoming train. The woman was killed, while the man lost his lower legs.