This video depicts SEPTA’s flotilla-like “wire train” renewing the catenary wire above the tracks on the R7 Chestnut Hill East line as it works its way past the Mount Airy train station. The wires become worn after decades of use. The workers are taking down old catenary wire, dropping and sliding it into the gondola for scrap metal. The new catenary wire is already in place and running trains. Naturally, for the safety of the workers, the catenary is de-energised and grounded.

The following description is from a SEPTA blog post of July 13, 2017

“In our world, a catenary is a system of overhead wires used to supply electricity to a locomotive, streetcar, or light rail vehicle which is equipped with a pantograph. The pantagraph presses against the underside of the lowest overhead wire, the contact wire.
Current collectors are electrically conductive and allow current to flow through to the train and back to the feeder station through the steel wheels on one or both running rails. Unlike simple overhead wires, in which the uninsulated wire is attached by clamps to closely spaced crosswires supported by poles, catenary systems use at least two wires. The catenary or messenger wire is hung at a specific tension between line structures, and a second wire is held in tension by the messenger wire, attached to it at frequent intervals by clamps and connecting wires known as droppers. The second wire is straight and level, parallel to the rail track, suspended over it as the roadway of a suspension bridge is over water.
Simple wire installations are common in light rail, especially on city streets, while more expensive catenary systems are suited to high-speed operations.
The Northeast Corridor in the United States has catenary over the 600 miles (1000 km) between Boston, Massachusetts and Washington, D.C. for Amtrak's high-speed Acela Express and other trains. Commuter rail agencies including MARC, SEPTA, NJ Transit, and Metro-North Railroad utilize the catenary to provide local service.
Overhead line equipment can be adversely affected by strong winds causing swinging wires. Power storms can knock the power out with lightning strikes on systems with overhead wires, stopping trains if there is a power surge. During cold or frosty weather, there is a risk of ice build-up on overhead lines. This can result in poor electrical contact between the collector and the overhead line, resulting in electrical arcing and power surges.
On the Media/Elwyn Line, we're working on replacing 17 miles of 80+ year old overhead catenary wire. We're also building/installing new catenary support poles.”