NYC subways slowly upgrading from 1930s-era technology

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.