UPDATE 1-Washington, D.C. subway to continue reduced service through at least Oct 31

(Adds more details, comments from briefing)

By David Shepardson

WASHINGTON, Oct 22 (Reuters) - The Washington, D.C.-area subway system said on Friday it will continue sharply reduced subway service through at least Oct. 31 as it works to finalize plans to return most of its railcars to service after an Oct. 12 derailment.

The Washington Metropolitan Area Transit Authority (WMATA) on Sunday was ordered to indefinitely remove about 60% of the subway system's railcars following inspections after the derailment.

The subway system, known as the Metro and which serves the U.S. capital and parts of Maryland and Virginia, has urged commuters to take buses or use other transit modes.

"We are working to restore full service on the system as quickly as possible but will only do so when it is safe to do so," said WMATA Chief Executive Paul Wiedefeld in a media briefing.

Average Metrorail weekday ridership has fallen about 25% this week to 157,000, WMATA said, down from an average of 213,000 last week excluding Monday's U.S. government holiday.

The Washington Metrorail Safety Commission (WMSC) said Sunday it had ordered WMATA's 748 7000-Series trains removed from service after the National Transportation Safety Board (NTSB) "identified safety concerns related to the spacing of wheels on 7000-Series railcar axles."

The NTSB on Monday said WMATA had been aware of wheel assembly issues since 2017 like the one that was involved in the recent derailment just outside Washington in Arlington, Virginia.

The NTSB said preliminary data showed that since 2017, there had been 31 WMATA wheel assembly failures, including 18 this year, and 21 failures were uncovered during recent inspections.

The Oct. 12 derailment did not injure any of the 187 passengers onboard, but NTSB chair Jennifer Homendy said the incident could have been "catastrophic."

(Reporting by David Shepardson Editing by Chris Reese)