The National Transportation Safety Board on Wednesday issued a safety alert to subway systems and commuter railroads over a wheelset problem that led to a derailment in the nation’s capital, saying the defect is difficult to detect and potentially devastating. The investigative agency released a preliminary report of its ongoing probe into the Oct. 12 accident involving the Washington subway system that ruled out other factors such as speed. The NTSB said the flaw in the rail car’s wheel and axle assembly is “not easily identifiable” with a routine visual check, and urged transit agencies to comply with a recent Federal Transit Administration directive to conduct inspections for misaligned wheels—something that NTSB Chair Jennifer Homendy urged early in the investigation. The NTSB broadened its alert to include commuter rail, which isn’t fully covered under the FTA order. Several transit and rail agencies have said they have made checks and the …