The two streetcars in this view appear to be open air summer trolleys, with cross bench seating and a running board for the conductor. These were popular for summer excursions and city tours, and these Pittsburgh cars may have been on a tour of the Forbes Street neighbourhood, which still looks upscale today. An example of such a car can be seen below.

That sloping lawn is pure decoration -- nobody could lie on it. Imagine being 6 and rolling down the slope to land Plop on the sidewalk! OW!

This appears to be the same building, now part of Temple Sinai at 5505 Forbes Ave.
If it is the same building, the street has been raised considerably.

UPDATE: No, it's not the same building.

The sign in the distance says "Vacuum Cleaner Co Pittsburgh". According to the 1908 Pittsburgh Business directory, there was a cleaning service called "Vacuum Cleaner Co" at 4614 Forbes. If that clue is accurate, the houses on the left are now Carnegie Mellon and this is the view:

After noting the three flights of stairs leading to the mansion on the left, I tried to imagine myself being tasked with mowing that lawn using a hand lawnmower. As a youngster in the late 1940s I mowed my family's flat lawn using a hand mower until I was about 15 years old so I am quite familiar with the state-of-the art in lawn maintenance, as it must have been in 1909.

When I was about 15 my dad loaned me money to purchase a power mower that I used to earn money mowing other lawns in our neighborhood, which was used to repay him. That's how we learned business sense in those days.

[1-hp mowers were not uncommon in 1909. - Dave]