Abstract: Fiber optic cables and similar signal conductors are installed in fluid transmission pipelines by cutting the pipeline at spaced apart points to form a section of pipeline for receiving the cable, installing pipeline pig launching and receiving fittings on the section of pipeline to receive the cable, and pumping a pipeline pig from the launching fitting to the receiving fitting while connected to one end of the cable by pumping liquid or gas into the pipeline behind the pig. Pressure responsive seals are provided for engagement with the cable at the launching fitting to prevent leakage of pumped fluid from the pipeline section into which the cable is being installed. The seals may be used to seal cable entry and exit points on inactive pipelines. Active pipelines are restored to service by the installation of a cable seal assembly having a split stuffing box and packing gland arrangement for sealing the entry and exit point of the cable with respect to the pipeline.