Based on what I've seen on some light rail lines like Sacramento, the point about low-speed running for simple trolley wire as on the top photo and higher speed needing the more substantial system of the second photo is correct. In town for street running simple suspension is used - probably easier to install and cheaper to maintain. On Sacramento RT the speed limit where that wire is used is 25 or 30 mph. Speed limit goes up to 55 on private r/w with catenary.

The double-wire in the second picture might be for higher current load as one post pointed out. That track does appear to serve a freight yard in the background. There might also be some kind of power section break there; on RT at the 29th St. station that happens, with closely parallel contact wires for a short stretch.