New York City subway chief makes resignation official

Bloomberg has maintained a high national profile during his tenure as New York mayor. He is leaving office next year after serving a twelve-year term.

Lhota potential bid to replace him follows him winning widespread acclaim for swiftly restarting subway service in New York City after the system sustained widespread damage during Hurricane Sandy.

The New York subway system is the most extensive and heavily used transit system in the U.S. It carries more than 2 billion each year on 24 subway lines that cover 659 miles of track and service 468 stations.

By comparison, the second largest public transit system in the U.S. is Washington, D.C.'s Metrorail system, which carries more than 200 million passengers per year. The DC Metro system has 103 miles and 86 stations.