NYC subways slowly upgrading from 1930s-era technology

In this Dec. 16, 2014 photo, lights on a section of the MTA subway interlocking switch and signal control board shows train locations, at the 4th Street MTA Supervisory Tower in New York. Transit officials are now replacing the 1930s manual signal system with 21st century digital technology that will allow more trains to travel closer together and a growing ridership to move around the city faster. (AP Photo/Bebeto Matthews) (The Associated Press)

This Dec. 16, 2014 photo shows manual levers on a section of the MTA subway interlocking switch and signal control board, in New York. Transit officials are now replacing the 1930s manual signal system with 21st century digital technology that will allow more trains to travel closer together and a growing ridership to move around the city faster. (AP Photo/Bebeto Matthews) (The Associated Press)

In this Dec. 16, 2014 photo, MTA train service supervisor Tweh Friday monitors a subway interlocking switch and signal control board, at the 4th Street MTA Supervisory Tower in New York. The 1930s technology of switches and relays requires a human operator to use hand levers to manually route trains and keep them separated at safe distances. (AP Photo/Bebeto Matthews) (The Associated Press)

The more than 6 million riders who take New York City's subways each day ride trains that still depend largely on a signal system that dates back to the 1930s.

Antiquated electro-mechanics with thousands of moving parts are still critical to operations. Dispatchers still control it all from 24-hour underground "towers." And they still use pencil and paper to track trains.

That eight-decade-old system is slowly being replaced by 21st-century digital technology that allows up to twice as many trains to safely travel closer together.

But only one line, the L linking Manhattan and Brooklyn, currently operates on new, computerized, automated signals.

Officials say it could take at least 20 years for the city's 700 miles of tracks to be fully computerized.