Something in this postcard caught my eye a few days ago, I like the tram lines and the starkness of the view and the almost symmetrical scene. Not having heard of Braddock, I googled and found that Braddock was the location of Andrew Carnegie's first public library (dedicated March 1889) in the US and I wondered if the building on the right was the library. Searching for an image of Belle Avenue in 1909 (when the postcard was sent), I think that it is actually the Belle Avenue High School which you can see here (scroll down to the second image which I think is a good likeness).
The postcard was sent in Nov 1909 from 'Uncle Harry' who had visited Chicago, was currently in Pittsburg and on his way to Niagara. That must have been quite a journey in 1909.
A bit of digging . . . .Uncle Harry was Harry Scopes, 'an inspection engineer' so maybe he was connected with the steel industry. 'Master Dickson', the recipient of the postcard, was his nephew Augustus William, born in 1904 and named after his grandfather (Harry's father). His mother, Helen was Harry's sister.