Sounds like a "hidden" washout could have been the cause, if there have been other recent landslides in the area. I forgot that Pgh has been getting a lot of rain lately. The soil under the tracks may have become so saturated that any train moving across could cause the rail to bend or shift. It can look solid one minute, then wash out the next.
Here's some video of a washout. Warning, some bad language. Many railroaders are notorious for cursing, haha.
Looks like they plan to have the tracks open sometime today. The accident is backing up trains across the NS system, as well as local commuter traffic.
Mess remains at South Side train derailment site
Are you Pittsburghers still getting rain there? If so, it could still make cleanup, etc. tricky. That whole mountainside there is probably still unstable.
Update: One track is open
One railroad track reopens after Station Square derailment