Is it possible to protect a subway system?

police officers work at the Parsons Green Underground Station after an explosion in London. Photo / AP

As the London Tube attack reminds us, public transit is a plum target for terrorists.

It's hard to secure (unlike airports, most cities can't set up massive checkpoints and bag scans) and easy to access. It's also, quite often, packed.

That might explain why there have been at least 387 attacks on trains, buses and passenger ferries in North America and Europe since 1970. South Asia has faced 1287 public transit assaults; there have been 801 in the Middle East.

Cities employ different security strategies.

Beijing shifts 10 million passengers daily. After a terrorist attack in western China in 2014, riders were forced to line up for a system that resembled airport check-in. Riders and their bags went through metal detectors.

London has pioneered anti-terror infrastructure. For example, the city has mostly done away with metal garbage bins, which could create deadly shrapnel if a bomb was planted inside. Israel makes use of metal detectors and X-ray machines at some bus stations. Buses, too, are bullet-resistant. Some also come with GPS tracking systems and video cameras so army officials can hear what's going on in an emergency.

In New York, extra police officers and state troopers patrol crowded transit stops. Suspicious packages are now regularly investigated and X-rayed and passengers' bags are subject to random searches.

But experts say bringing airline-style security to US subways would be virtually impossible, and the above measures would be useless against suicide bombers.