This is an excellent idea, and I believe it would benefit the city and metropolitan area immensely.
Pittsburgh's unique geography makes it difficult to expand the highway infrastructure in response to increases in population and traffic demands, leading to heavier congestion, longer commutes, and more wear and tear on the existing roadways. Cities with flatter and more open topography, such as Houston, Dallas, and Phoenix, are able to simply build wider highways and new alternative routes. Pittsburgh needs a different solution, since there simply isn't physical space to expand or construct in such a manner.
All these hundreds of miles of unused track are still viable with some rehab and minor repair, and would have a far more profound impact on the area's economy and environmental outlook than alternative uses, such as the "Rails to Trails" program. While that was a step in the right direction, it came at a significant cost to the municipalities involved, and offered no tangible economic benefit, only aesthetic improvement.
Converting the dormant railways to an integrated light rail and dovetailing with the existing T system would create a strong and sustainable transportation network and put Pittsburgh on par with many larger cities, such as Chicago and San Diego, where the light rail is a necessity and solution for many daily commuters.
Having the easements and physical tracks already in place makes this proposal a simple matter of consolidation and rehabilitation, with a far lower capital outlay than would be required if starting from zero. Stations wouldn't need to be elaborate, simple concrete and metal platforms and entry systems would suffice.
In terms of value to the taxpayer, rail commuting would significantly reduce the cost of the daily commute by eliminating the need to park in the city, saving vehicle fuel and maintenance expenses, and allowing a more predictable commuting timeline, which would translate to more time at home with the family. The additional benefit from fewer cars on the roads and highways would be reduced municipal and state maintenance costs, freeing up precious budget resources for other essential services.
I'm 100% in favor of this idea, and would love to see it become reality!