BOSTON -- Trains carrying toxic chemicals roll through Massachusetts every day, but Team 5 Investigates has uncovered a shocking lack of knowledge about where they're going and what's on board.

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Though it doesn't happen often, when a derailment or a collision involves a tanker carrying explosive chemicals, hazardous waste or what one federal official calls a "you breathe, you die" gas, panic sets in, neighborhoods clear out and people die.

Now, the stakes are even higher.

"Chemicals on trains are near the top of the al-Qaida terrorist target list," said Rep. Ed Markey of Massachusetts. "These are mobile 'hell on wheels' if anything happens."

Just ask Bill Snow who has lived 75 feet from the tracks in Charlestown for decades.

Four years ago, highly toxic hydrochloric acid leaked from a tanker, evacuating residents for days. Every night since then, Snow hears the rumble of trains just yards from his bed, but what frightens him more?

"If you don't wake up," Snow explained. "The last time, they caught it. This time, it might be something much more catastrophic."

In Lawrence last May, a derailment led to a collision with a tanker carrying sodium chlorate that spilled and caught fire. Families evacuated.

Fire officials rarely know what they're dealing with until they arrive on scene.

"This hazardous material is coming into Lawrence," said Team 5 Investigates reporter Janet Wu.

"Right," said Lawrence Fire Chief Peter Takvorian.

"Leaving Lawrence," Wu continued.

"Right," he said.

"Going through crowded places," she said.

"Yes," he said.

"You have no idea when it's coming or what is being brought in," she said.

Local communities have little information and no control over the railroads yet their fire departments are first in line to respond if there's a disaster.

Instead, first responders rely on numbered placards --- forcing them to get close enough to read it without jeopardizing their own safety.

The Federal Railroad Administration insists these emergencies are rare.

"There are more than 800,000 shipments of hazardous material every day," said Bill Schoonover of the FRA. "And so it becomes very difficult to provide all that information to every community to which these materials go through."

Carol McCarthy lives near the tracks in Lowell.

"On a weekly basis, you can find them parked there," she said. "As much as 48 hours, two to three days."

She took these photos of tankers loaded with a flammable gas behind a neighbor's house --- so close that seconds would make the difference between a crisis and a catastrophe.

"I would love to see if the rail vehicle is going to be parked overnight, that it is communicated verbally to maybe the dispatch center or the incident commander that's on-duty," said Lt. George Rose of the Lowell Fire Department.

Right now, first responders can request a list of the most common hazardous materials hauled through their communities. But railroads are not required to provide the information. And the first responders we spoke with said many don't.

"We cannot operate in a post 9/11 environment on a voluntary basis," Markey said.

Two years ago, Markey pushed through legislation to strengthen containers carrying hazardous materials, to begin training programs in the event of a crisis, to purchase new equipment and to find new routes for the most common yet dangerous chemicals like chlorine, propane and ammonia.

Yet the most critical piece is missing.

"Will you avoid the most densely populated areas?" Wu asked.

"Not necessarily," Schoonover said.

Right now, the rail companies are only required to ask communities for advice on the safest routes. They don't have to take it.

"The industry doesn't want to have to spend the money to protect the public," Markey said, "even though they're bringing these highly toxic dangerous chemicals through communities that are highly populated."

What worries many are the vulnerability and accessibility of tankers parked in rail yards, parked next to apartment complexes, sitting just a few feet from homes for days at a time.

Team 5 Investigates found easy access to the same rail yard where last spring's hazmat emergency in Lawrence took place. For nearly an hour, we walked the yard, shooting video undisturbed.

"There are nightclubs in Boston that are harder to get into than railway yards with dangerous chemicals," Markey said.

"This is our neighborhood," McCarthy said. "If they have a right to park it there, then we have a right to know that we can be safe."

"It doesn't take much for an accident to happen," Snow said. "And with the way things are today, it's even more of a danger."