When a series of smoke bombs paralyzed Montreal's subway service last week, the mayor asked the public rhetorically what they wanted him to do.

"We've put in place whatever we had to do," Gérald Tremblay said. "Do you want me to take 4,600 policemen and women and put them in the subway? You want me to close the subway? What do you want me to do?

"Are we going to stop living because we have a crisis on our hands? No. What's the solution? That's the question."

Montreal's transit authority responded to the incident by increasing the presence of security officials and asking its employees to be more vigilant about suspicious behaviour.

But security experts say there are other ways of beefing up security, and that passengers are an important key to improving safety.

"We know that alert passengers as well as alert staff play a major role," says Brian Michael Jenkins, director of the national transportation security centre at the California-based Mineta Transportation Institute.

As evidence, four suspects have since been charged in connection with the Montreal smoke bombs. Some had been identified via eyewitness pictures taken by commuters during the incidents.

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Police handed out this photo of suspects taken by commuters. (HO-Montreal Police/Canadian Press)

Jenkins said that getting passengers involved is a much more effective approach since applying airport security standards to public surface transportation is not realistic. The volumes of people on public transit are overwhelming, the costs would be prohibitive and the delays involved would essentially destroy convenient surface transportation.

"People may be willing to wait in line to undergo security at airports or cross-country flights. But to impose that kind of wait on a system where you take a 15-minute, 20-minute ride two times a day is not realistic."

Installing more closed-circuit television cameras, increasing the presence of security, both visible and undercover and adding bomb-sniffing dog teams are all options that security experts sometimes recommend.

As well, some systems have gone to random passenger screening, Jenkins said. New York, New Jersey, Washington, Boston and Los Angeles all have some variation of this procedure, which is voluntary. But Jenkins stresses the importance of getting riders involved in their own security.

According to statistics gathered by the institute, around 15 to 16 per cent of attempted bombings against surface transportation around the world were thwarted because riders or staff saw a suspicious object and notified authorities.

"So that indicated a fairly significant role, and I would assert an even greater potential, to engage the public on their own security," he said.

For example, during the IRA bombings of London transit, passengers became part of the security system.

"The people in London got so good at this that authorities could depend on the fact that a suspicious object would be  identified and they'd be notified in a couple minutes."

Eventually, Jenkins said, the IRA, which had been selecting high-profile transit targets, had to move outward to the suburbs.

Marianne Rouette, a spokeswoman for the Montreal transit system, said that passengers can use the telephones on the platforms to alert authorities in emergencies, push a button on the train to speak to the operator or call 911.

Jenkins said that in order to engage passengers effectively, they need to know how to communicate what they have seen.

"If people are just exhorted to be vigilant ... nothing happens, then people stop doing it."

"It is essential to provide some systematic easy way of communicating. Who do I tell, how do I tell them, where do I go. Do I dial 911 or what?"

Also, some kind of closure must be achieved, he said.

"So somebody has to tell the [passenger] 'We’re on it,' and people see something happen."

Jenkins said transit officials have to provide training or instructions.

"This is what we're talking about,  this is what we're looking for. This is what you want to think about."

At the same time, he said, this shouldn't be about "turning people into a street corner posse looking for what they think are shifty-eyed terrorists."

A number of transit systems in the U.S. have adopted a "see something, say something" approach, Jenkins said. Many are expanding that initiative so people can very quickly utilize their smartphones to take pictures or send emails to the proper authorities regarding suspicious activity.

Since the phone gives up its location, security officials can quickly zoom in on the area in question.

Passengers have an advantage over outside security people because they know their environment better, as they tend to ride the system at the same time every day, see some of the same people and are more sensitive to suspicious behaviour.

"When something is off, something is not right, something is out of place, something is not where it should be, they can pick it up faster than anyone else," Jenkins said.

"If those charged with security can create the machinery that can tap into those additional eyes and that knowledge, that significantly enhances the security of the system, it engages the public and it is the least controversial of all the security measures that one can impose because in fact those being protected are those engaged in protecting themselves."