That question has confronted cleanup crews since seven of 101 cars on a CSX train derailed south of Center City on Jan. 20, in the middle of the night. The slow and delicate process of removing them - two of the cars were leaning off the bridge - has thrown jackhammers and 100-ton cranes into action, and sent dozens of workers to cringe-worthy heights above the icy river.
The accident came at a time when federal safety officials were already voicing concerns about the risks of moving flammable crude oil around the country by rail. Just last month, a trainload of crude in North Dakota exploded, causing millions in estimated damage. The Philadelphia derailment was far less serious - no fire started, no oil spilled. The workers' task was to keep it that way.
Derailment experts from as far away as Mobile, Ala., have worked in 12-hour shifts for more than a week at the site, located near the University City hospital complex. The last freight car on the bridge was expected to be moved Tuesday night.
Thousands of drivers on the Schuylkill Expressway have been able to see the hard-hatted workers, about 70 during the day and 40 at night, who had a dizzying view of the road and the river from above.
How high? About 35 to 40 feet. "Enough to make me uncomfortable," said Harry Hopes, a manager of hazardous materials for CSX who makes his home in Charlotte, N.C., and has nearly four decades' experience fixing derailments - this being his highest.
Workers like Hopes wore up to six layers at a time, wrapping themselves in safety harnesses, carefully walking across planks that separated them from a significant plunge.
Their first job last week: Stabilize the two leaning cars with cable rope and chains. Then came the messy part: Pumping out the crude oil contained in six of the seven train cars.
Typically, the oil flows quickly - almost like water through a pipe. But when temperatures are in the teens, the crude thickens. A pump that can move nearly 400 gallons a minute is slowed to about 100.
Then came the sand mixed with concrete - nearly 50 tons of it - that filled the last train car, the "buffer car" that adds stability. Material that's so heavy, crews needed jackhammers to cut through it.
After the sand removal, it took about an hour for two 100-ton cranes to pull that car from the bridge, over an expressway entrance ramp and into a wooded area next to the railroad tracks. "Slow pace," one worker said. "Like molasses."
Hopes, the veteran derailment expert, turned 58 Tuesday. He's living out of a Holiday Inn and trudging across a snowy railroad bridge 540 miles from home to help with the project.
"I have never spent a birthday like this before," Hopes said. "And hopefully I won't have to spend another."