Then the second engineer noticed strange movement in the overhead catenary wires above the eastbound train, and hit the emergency brake on his train, Mr. Cahill said.
Contractors have been replacing catenary wires the overhead electric cables that power trains in various spots along the line since 1991, and in some cases replacing original equipment erected in 1907.
The gap between the pantograph, which is the arm on top of the train car that draws power from overhead lines, and the catenary system must remain within a precise distance and tension.