CNN — A former Washington, DC, Metro transit system contractor retained access to “critical and sensitive” Metro data from his computer in Russia after he left his job in a breach that raises broader security concerns about one of the nation’s largest transit systems, according to a report released Wednesday. The cybersecurity vulnerabilities of the Washington Metropolitan Area Transit Authority are “a cause of grave concern” that the authority’s networks are “at unacceptable risk” of hacking or other forms of compromise, a report from the WMATA’s inspector general concluded. It’s the latest in a series of warnings from auditors over…