Seven people were hurt -- none seriously -- in the crash, which occurred about 2:30 a.m. Saturday.
On a sunny Memorial Day afternoon, at least seven backhoes, cranes and endloaders operated simultaneously to clear debris.
Workers from R.J. Corman Derailment Services and Budrovich, a heavy equipment company, coordinated efforts. The fruit of their labors was readily apparent: A pile of twisted metal taller than the top of a backhoe sat next to a large, open area where a section of track had been removed.
The bridge decking had been cleared away, and rebar protruded from the broken concrete remnants of each end of the collapsed overpass.
More than 20 cars in various states of damage were lined up along the right of way next to the track, and upended rail cars lay along the shoulder of Route M leading to the overpass.
Official information was scarce Monday, but a worker at the scene estimated at least 20 different contractors had been on the scene at one point.
The overpass collapsed after a Union Pacific train struck a Burlington Northern Santa Fe train, derailing several rail cars and knocking down one of the bridge's support pillars in the process, the National Transportation Safety Bureau reported.
Mark Davis, director of corporate relations and media for Union Pacific, referred questions to the NTSB.
Williams said investigators had gathered the information they needed from the crash site.
"We've gathered the information that we need in terms of our documentation, and we've turned it back over to them," he said, referring to the two railroad companies whose trains were involved in the collision.
Robert Sumwalt, an NTSB board member, has said it could take up to a year for investigators to analyze the information and reach a conclusion.
In a Sunday news conference posted on ntsb.gov, Sumwalt said the conductor and engineer on the BNSF train initially were unaware it had been hit.
"For the first few minutes after the collision, all the BNSF crew was aware of was that their train had gone into emergency braking," which happens whenever a train loses air pressure, he said.
Sumwalt said the conductor told investigators he thought an air hose might be loose and discovered the aftermath of the collision when he exited the train to see why it was braking.
He declined to speculate on the reason for the collision, saying the cause was still under investigation.
"What we can say factually is that the BNSF train was struck about a dozen cars back," Sumwalt said. "How did that happen? Why did that happen? That's what our investigation is trying to find out."
Investigators downloaded signal records, photographed and measured the bridge, and recovered camera data from locomotives, he said.
A camera on the BNSF train's third locomotive -- which was facing backward -- gives a clear view of the bridge, Sumwalt said.
The bridge, which is 367 feet long, was constructed in 1988 and was most recently inspected Feb. 25, when the Missouri Department of Transportation rated it a seven on a scale of one to nine, Sumwalt said.
The Federal Highway Administration considers a seven to be good condition, he said.
Two of the five spans on the overpass collapsed after a rail car hit a support column, Sumwalt said.
"It had a rail car basically wrapped around it in somewhat of a 'U' shape, and it collapsed as a result of that collision," he said.
According to an Associated Press report, a MODOT engineer estimated the bridge replacement cost at $3 million.
"The damage is very extensive," Mark Shelton said. "We're going to end up removing the entire bridge and completely replacing it."
He said the bridge is expected to reopen in early September.
Route M near Rockview, Mo.