Transcript for How authorities are protecting US passengers on one of the busiest travel weekends

First, increased concerns about railway safety as millions of Americans head home after the holiday. The new fear this morning is that terrorists may target America's extensive rail system. Here's ABC's David Kerley. Reporter: This surveillance video capturing two would-be terrorists. The third man, his face covered by the digital circle is an FBI agent. The two men are serving life sentences. Train rg still a terror target. Resulting in a recent intelligence notice being sent to local authorities. This is a serious warning. For TSA to send out a warning to law enforcement says we're concerned about this tret. Reporter: The concern came in Al Qaeda's latest issue of its "Inspire" magazine, imploring followers to target trains. Build a tool to instill fear. The tracks are the target. This drone was the immediate reaction in Philadelphia to that federal notification. The transit police chief on his own launching this new technology. Special operations teams are using that drone with a high-resolution camera to check some of the 2200 miles of track for any potential threats. This is a great way for us to patrol. A track. Without walking on the tracks. Reporter: While federal counterterrorism experts gather intelligence, it's up to local authorities to guard train stations and the rail lines. Can this drone make a difference? When somebody actually decides they want to launch an take, these folks are going to be the once that prevent it from happening. Reporter: The U.S. Rail system stretches over 140,000 miles of track. Much of it unsecured. Are we doing enough to protect our rails? We can do more. It's impossible to secure every single inch of the rail. Reporter: Counterterrorism sources say while this concern is not new, this latest threat by Al Qaeda is a reminder that our rails remain a target. David Kerley, ABC news, Washington. A timely story for sure. 140,000 miles. That's a big area. Big target. Big thanks to David Kerley this morning.

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