Here's a beautiful, small tree whose name escaped me until Dianne Machesney sent this picture.
This is American Fringetree (Chionanthus virginicus).
I should have been able to guess its name because of its delicate fringe-like flowers.
Pennsylvania is the northernmost edge of Fringetree's range and it's even considered "threatened" in the state, but it's so pretty that landscape designers plant it as an ornamental. I'm sure that's why I've seen it in Schenley Park. Dianne found this one in North Park.
Keep an eye out for it. It's worth a look.
(photo by Dianne Machesney)
p.s. UPDATE, MAY 22: Fringetree is now blooming in Schenley Park.