The upper photo is what is called single suspension trolley wire. The copper trolley wire is supported directly by cross wires.

The lower photo is what is called catenary suspension. The upper wire is stranded high strength steel wire that is not a great conductor of electricity. The copper contact wire is suspended by hangers from the upper wire.

The big advantage of catenary wire is that the contact wire can be absolutely level and straight allowing high speed operation. Single suspension wire droops between hangers. The pantograph or trolley pole must go up and down to track the wire. This limits high speed operation. Although there were exceptions, typically single suspension wire was not used where operating speeds were over about 40 mph. Speed of the South Shore car on a city street in the photo would have been limited.