BOSTON -- The deadly terrorist attacks on trains in Madrid in 2004 and on London’s Underground in 2005 highlight the vulnerability of the rails as a target.

Yet in the United States, spending on aviation security far exceeds the amount of money spent on security for the public transportation system.

It’s a fact South Shore salesman Rick Dacey wasn’t aware of, but it confirms his concerns after a recent Amtrak trip from Boston to New York City.

“When I got onto the train nobody checked any kind of bags,” said Dacey. “No computer checks. I was very surprised compared to the security measures at the airlines.”

News of a recent alleged terrorist plot against the New York City subway system worries Dacey, too. “That concerns me and it should concern the rest of the citizens.”

Team 5 Investigates Kelley Tuthill reported Sunday that more than 26 million passengers travel through Logan Airport each year, slightly more than the 20 million who ride the MBTA during the average month, according to the MBTA transit police chief.

But the American Public Transit Association told Team 5 Investigates that more than $30 billion have been spent on aviation security since the terrorist attacks on Sept. 11, 2001, compared to just $1.5 billion for security on public transportation.

“That translates to less than four cents on the dollars for public transportation versus aviation security,” said MBTA transit chief Paul MacMillan. “Terrorists have attacked mass transit systems overseas. It's clear it is a vulnerable target and we have to do all we can to protect it.”

The MBTA performs random explosives tests on passengers’ bags and has increased the number of surveillance cameras from fewer than 100 to more than 800 in recent years.

Team 5 Investigates used that network of surveillance cameras to test another key part of MBTA security -- the riders themselves.

Announcements to riders urge the public to be watchful. With the cooperation of transit police, Team 5 Investigates watched what happens when a backpack is left unattended in a station.

The MBTA put the bag right in the middle of busy Back Bay station where people buy their Charlie cards. It was also in front of the fare gates where people come on and off the trains

At the height of the morning rush, hundreds of commuters walked right past the unaccompanied backpack. Others stood right next to it and said nothing. Ten minutes went by before the first of just a handful of passengers alerted nearby MBTA staff.

“I just knew it didn't look like it belonged there,” said one man who spoke up during our hour-long test.

“Some people going to work they don't look. They don't care,” said another rider.

Riders should care, according to Harvard University preparedness expert Leonard Marcus, who rode the MBTA with Tuthill.

Pointing to a uniformed officer, Marcus said, “This station is safe because he's here. He doesn't have to do a thing.”

But security personnel cost more money than passive surveillance systems.

“We know in other countries, for example, London,” Marcus said, “people are more attuned to looking for suspicious packages and for alerting officials.”

“If right now there were a suspicious package right there,” Marcus said, “we would ask does this belong to anyone. No? Everyone move to other side of the car, and open the windows if possible.”

And, Marcus advises passengers to keep their eyes open for other riders who are acting suspiciously.

“If something happens that's out of the ordinary, that's the time to press the button in your mind and say I need to keep an eye on this.”