[WASHINGTON] Washington's metro train system will shut all day on Wednesday for emergency inspections after an electrical fire in a tunnel, transport chiefs said, in an unprecedented move that threatens to cause mass disruption in the US capital.

It is the subway's first system-wide closure for non-weather-related conditions, Metro chairman Jack Evans said Tuesday, admitting it would likely trigger transport misery for hundreds of thousands of commuters and tourists.

The metro system - the second busiest in the United States after New York - serves about 700,000 customers a day in Washington and the neighboring states of Maryland and Virginia, where many commute from into the capital.

The closure was to begin at midnight (0400 GMT Wednesday).

"We will suspend rail service for 24 hours to inspect 600 underground jumper cables. The entire rail system will be closed until 5.00am Thursday," Paul Wiedefeld, general manager of the Washington Metropolitan Area Transit Authority, told a news conference Tuesday.

It is the latest bad publicity for the Washington metro, which has 91 stations and began operating in 1976.

In January last year, a woman died and dozens of people were injured after thick smoke filled a metro tunnel during evening rush hour due to an electrical malfunction.

The whole system - along with all buses - also closed for a whole weekend during the "Snowzilla" blizzard that bombarded much of the US East Coast in January.

Then on Monday this week, a cable fire erupted in a tunnel, sparking major delays on three lines - and ultimately the decision to shut the system down.

An investigation was still under way, Mr Wiedefeld said, but "commonalities" were found between Monday's fire and the deadly January 2015 incident.

"While the risk to the public is very low, I cannot rule out a potential life-safety issue here and this is why we must take this action immediately," he said.

"When I say safety is our highest priority, I mean it. That sometimes means making tough, unpopular decisions and this is one of those for sure.

"I fully recognise the hardship this causes to the region and to the community." Mr Evans admitted that the decision would cause "a lot of congestion." "It's going to be very difficult, we recognise, for people to get in to work, for school children to get to school, for everyone who relies on Metro.

"But safety is paramount and we need to get to the bottom of this." The city's public schools were to remain open on Wednesday.

"We are working with Metro to add additional bus service," officials said on Twitter, adding that late arrivals and absences would be excused.

Many in the US capital rely on the metro, which has six colour-coded rail lines, to get to work or visit tourist sites.

But residents frequently complain about delays, long waits, station closures and weekend disruptions caused by scheduled track work.

AFP