The train derailment and fire in the heart of Baltimore’s downtown Wednesday alerted the public to an open secret among those in the know: Every day, by rail, by truck and by ship, hundreds of thousands of gallons of hazardous chemicals pass through the city.

But no one -- not even those who would have to respond to an accident -- knows what dangerous materials are crossing the city at any given time, though many shipments carry the potential for disaster.

For instance, the train that derailed two days ago might have been transporting the gas used in death chambers -- hydrogen cyanide -- a not uncommon shipment on Baltimore’s rails.

Or highly flammable propane, perhaps the most typical hazardous cargo in this area. Or chlorine, which could send a toxic plume more than 25 miles downwind if a single 90-ton rail car ruptured.

Federal agencies regulate the types of containers that hold these chemicals, the safety devices and signs on them and, in some cases, each car’s proximity to other chemicals on a train.

But no one monitors the types and quantities of chemicals passing through Baltimore or anywhere else in the country. And no agency requires that communities be forewarned of such shipments.

It is an information gap that citizens groups have long fought to close in the interest of better emergency planning.

"There is no federal agency that does a good job of tracking hazardous materials," said Paul Orum, director of Community Right to Know, a Washington-based group. "People have a right to know if they can be hurt or injured by materials on rails or roads or on barges."

Manufacturing plants and other facilities that keep hazardous chemicals on site must draft a worst- case accident scenario to let communities know the potential consequences for nearby schools, hospitals and neighborhoods. But no similar requirement exists for chemicals in transit.

"As with so many of the environmental laws, these are the kinds of issues that come to public attention in response to disasters," said Bradley Campbell, former Mid-Atlantic administrator for the Environmental Protection Agency. "This is not one where there is a lot of attention, in part because the safety record, particularly for rail, has been pretty good."

Maryland has experienced nearly 5,000 hazardous spills during transit over the last 30 years, with the annual number increasing over the past decade. About 3,500 occurred on highways; only 217 on railways.

Federal rail officials said Thursday that 2 million tank-car loads of hazardous materials were shipped nationwide last year, with 35 train accidents resulting in the release of dangerous chemicals.

Alerting communities in advance, they said, could have a dangerous result -- inadvertently informing someone interested in sabotage or terrorism.

"The key is to get the appropriate information to the emergency crews as soon as it’s determined there is an emergency," said George Gavalla, safety director for the Federal Railroad Administration.

Within 15 minutes of being called Wednesday, the Fire Department knew the contents of the train, including caustic acids that can cause severe burns and lung damage.

Still, such information falls short, said Assistant Chief Michael Dalton. "What the manifest doesn’t tell you is what can happen if the tankers rupture and the chemicals mix," he said. "Then the whole picture changes. It can create its own witches’ brew and there wouldn’t even be a chemical name for it."

The fire marshal is routinely notified about the shipment of explosives, Dalton said. "But a lot of chemicals are far more hazardous under the right conditions than a boxcar full of dynamite."

In this case, the risk of a lethal mix was low because most of the chemicals involved are acids, rather than a heat-generating and potentially explosive mix of acids and alkalis, officials said.

For the century-old, 1.7-mile Howard Street Tunnel, there is no restriction on the types of chemicals that may be transported through. About two dozen trains pass through it daily, according to CSX; a similar number, including Amtrak passenger trains, use a 1.3- mile tunnel west of Pennsylvania Station.