WASHINGTON (CNN) -- President-elect Barack Obama's January 17 train trip from Philadelphia, Pennsylvania, to Washington is intended to make the inauguration the most open and accessible in history.

President-elect Barack Obama's train ride from Philadelphia to Washington will pose myriad security risks.

But it is also presenting the U.S. Secret Service with security problems. Miles and miles and miles of them.

In addition to the well-publicized "whistle stops" in Philadelphia; Wilmington, Delaware; and Baltimore, Maryland, the Presidential Inauguration Committee says the public will have the opportunity to view the train at other locations along its 137-mile route.

But the committee has not indicated where those sites will be, and the Secret Service has yet to release what security restrictions will be in place.

Security experts say the train ride presents traditional threats to the VIPs on board, as well as countless buildings, homes and warehouses along the route. And there are non-traditional vulnerabilities: scores of bridges and tunnels that could be sabotaged.

And, two environmental groups have warned, terrorists could take a page from al Qaeda's playbook, using existing infrastructure, in this case chemical plants along the route, as an attack method.

In a letter to the Secret Service, Greenpeace and Friends of the Earth wrote that they were worried that security efforts focused on the Obama entourage "might not extend to the larger community which may suffer serious consequences in case of even one moderately successful terrorist release of ultrahazardous chemicals."

"We will be urging Mr. Obama and [Vice President-elect Joe] Biden to re-consider" plans to travel by train, the groups wrote.

In response, Secret Service spokesman Eric Zahren told the groups, "Please be assured, the U.S. Secret Service is working closely with federal, state and local agencies to maintain a safe and secure environment for all of our protectees."

Homeland Security Secretary Michael Chertoff, whose department includes the Secret Service, said he is confident of the agency's abilities to protect the president-elect.

"Remember, the Secret Service has taken the president to Iraq and to Afghanistan, to some very hostile places around the world. I'm comfortable that we have the skills, working of course with state and local authorities, to protect the president-elect in Pennsylvania and Baltimore and Washington, D.C.," Chertoff said.

Chertoff said he is aware of the environmental groups' concerns.

"I suspect it's being done, frankly, largely as a public relations ploy," he said. "I don't think there's any particular threat from chemical plants along the train route." iReport.com: Send your photos from the train stops

A former Secret Service agent said the agency has had experience with presidential train trips and said authorities typically install chemical, biological and radiological detectors along the route.

Amtrak and the Secret Service will not say whether Obama's train car will be armored, though experts say special cars have been used in the past.

"Security will be provided in the air, on the ground and in the water," said Amtrak Police Department Chief John O'Connor. "It's a daunting challenge, but there are many, many police departments that are working together to make this happen."

Once the train reaches its final destination, the presidential inauguration itself will offer an attractive target for terrorists because of "the historic significance of the nation's first minority president," a U.S. intelligence assessment said.

The report hastened to add that no specific threat has been detected thus far.

The document says the swearing-in of the first minority president "increases the potential threat, particularly stemming from individuals on the extremist fringe of the white supremacist movement." But it adds that no such groups are known to have threatened the inauguration so far.

The report further noted that "lone offenders" not connected to organized groups pose the greatest potential threat because such offenders are often the most difficult to detect.

The document says there have been some reports of lone offenders making threats but no indication that any of the potential threats "have progressed past rhetoric to operational planning."

CNN's Carol Cratty contributed to this report.

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