In the United States, 11 million Americans use the subway to get to work or school every day.
So the terrifying images of Tuesday's coordinated bombing attacks in Mumbai, India, that killed 183 people and wounded more than 700 have many mass transit commuters asking, "How safe are our trains?"
At the Jersey City, N.J., commuter train station, the Department of Homeland Security is launching a test passenger screening program.
The screening program was already in development, but the India attacks made it more urgent.
Other U.S. subway systems have also been placed on alert.
In New York City, more police officers are riding the rails and searching passengers' bags.
There had been an extra police presence since last week's revelation that a terrorist plot against New York-area trains and tunnels had been foiled.
In Houston, extra officers patrolled the trains; Atlanta already had them in place. San Francisco and Seattle raised their alert levels. Most cities say the steps they took after the London bombings last year were still in place.
Several Devices Being Tested
New Jersey transit officials are testing seven different devices to see which technology is best at spotting bombs and potential suicide bombers.
One looks -- and works -- very much like an airport screener.
It screens a person's body as he walks through, creating a kind of chart of the layers of clothing he is wearing.
Security workers will see this on a screen, and hopefully detect whether someone has a bomb or a suspicious lump under his clothing.
Another device, which looks like a circular phone booth, is then used for a closer look.
If something looks suspicious, a person would be told to enter this booth.
It revolves around him, creating an even more detailed image of the person's layers of clothing and any other items he may be wearing.
Again, security workers will look for lumps, belts, anything that could be a bomb.
The testing devices are part of a $10 million project funded by Congress that is aimed at helping the Department of Homeland Security implement the best possible rail security system.
The goal for now is to simply collect data. Security officials want to see how the systems perform, and they will pass the results along to Congress and the machines' manufacturers.
What to Do in an Emergency
In the meantime, it's important for regular riders to know what to do in an emergency.
Transit officials have several tips on staying safe:
Unless you're in immediate danger, stay on the train and wait for instructions. If you don't hear an announcement, try to contact the conductor by intercom.
If there's danger in your car, try to move to the next car rather than exiting the train.
If you're facing imminent danger and can't get out the door, you can often push out a window.
If you exit the train, walk toward the fresh air. Look for blue lights that mark emergency call boxes, and let rescuers know you're in the tunnel as soon as possible.
Each transit system is different. Familiarize yourself with your city's transit system, especially where the third rail -- which poses an electrocution danger -- is located.
The Red Cross has safety kits that contain a dust mask, a light stick, a whistle to call for help, and a packet of water.
You can also achieve some of the same results in an emergency by putting your shirt over your face, using your cell phone as a flashlight, and yelling or clapping to get rescuers' attention.