Boeing is braced for temporary disruption to its busy 737 production system in Renton after a train carrying six fuselages from Spirit Aerosystems in Wichita, Kansas, derailed in a mountain pass, sending several freight cars into a river.

The accident, which occurred at Rivulet, some 18 miles east of the Montana town of Superior on July 3, derailed 19 cars in a westbound train.  According to the rail company, BNSF, three of the cars carrying 737 fuselages slid down a steep embankment and came to rest in the Clark Fork River.

Boeing says the train was carrying six 737 fuselages as well as assemblies for the 777 and 747. It adds that personnel have been deployed to the scene “to begin a thorough assessment of the situation.” The cause of the derailment remains under investigation but rail officials expect the line will reopen over the July 5-6 weekend.

The incident will inevitably impact the production flow at Renton which since March has been assembling 737 at the rate of 42 per month. Every fuselage for the production line is built by Spirit in Wichita and is transported to Renton by rail. Several 737 fuselages are in transit every day at various locations along the line from Kansas which has ramped up shipments to keep pace with production increases at Boeing. Since 2010, production has risen about 33%, from 31.5 to 42 aircraft a month, and in 2017 is scheduled to increase to 47 a month.

Spirit AeroSystems also builds the nose section, body panels, wing center section, nose gear door and nacelles in Wichita for the 747-8, as well as the forward nose section for the 777. Boeing has not specified which of these assemblies was on the derailed train. Production of the 747-8 currently stands at 1.5 per month while the 777 is at its highest rate ever at 8.3.