An investigation by the National Transportation Safety Board (NTSB) found that the crew of the MARC train had forgotten the indication of an approach signal which they had passed before a station stop, and as a consequence, could not slow down in time after encountering a stop signal.

Amtrak operates the single daily Capitol Limited, a Washington–Chicago overnight train, over the route as well, though it makes fewer stops.

The National Transportation Safety Board (NTSB) described the conditions that day as a "blowing snowfall", with a 5-inch (130 mm) accumulation.

[3]: 1  The train consisted of EMD GP39H-2 diesel locomotive number 73, two passenger coaches, and cab control car no.

29, the Capitol Limited, departed Union Station at 5:25 pm, traveling westbound towards Chicago.

286 did not obey this restriction, and, after departing Kensington, the train reached 66 miles per hour (106 km/h) before the crew applied the emergency brakes.

[4] The deaths of all three MARC crew members in the collision meant that the reason for the failure would remain unknown; in its report, NTSB ascribed it to "the apparent failure of the engineer and the train crew because of multiple distractions to operate MARC train 286.

"[3]: vii  The focus of the investigators and the public shifted to safety systems that could have prevented the crash and the design of the commuter rail cars themselves.

One of the crew members and seven of the eight MARC passengers who died were killed not by the collision itself but by a fire that started when the exposed diesel fuel tanks on the Amtrak locomotive ruptured.

[3]: 73  The NTSB also strongly criticized CSX and the Federal Transit Administration for the removal of a signal between Kensington and Georgetown Junction as part of capacity improvements on the Metropolitan Subdivision.

The NTSB argued that having this signal after Kensington would have reduced the likelihood of the sort of human error that caused the crash, and in the NTSB's view, CSX and the FTA did not properly assess the effects of removing the signal.

[3]: 53–63 Following the accident, the engineer and conductor of the Capitol Limited filed lawsuits against Amtrak, CSX, and the state of Maryland for $103 million (1996 USD) alleging negligence[needs update], singling out the removal of the signal between Kensington and Georgetown Junction and the operator error by the MARC engineer.

Both men claimed that the injuries they sustained in the crash "prevent[ed] them from returning to work".

The new rules required new control cars and multiple units to be built to higher crashworthiness standards.