The Connecticut Southern Railroad was founded in 1996 to operate cast-off Conrail lines in central Connecticut, ending up operating over approximately 90 miles of line (primarily along the Amtrak-owned trackage between New Haven & Springfield, MA.
The P&W purchased it from Railtex in 2008 (the P&W wanted the bridge between Hartford & East Hartford, but Railtex would only sell the railroad as a package deal) to connect to the PV&T’s Connecticut River line via said bridge.
The CSO came with a bunch of EMD locomotives, which were promptly transferred to the Diesel Locomotive Pool and replaced with a handful of (leased) DL30s, which were then replaced with P&W power and the class 273 crocodile when wire was strung over the New Haven to Springfield line (a project that had Amtrak & ConnDOT’s enthusiastic approval, because they’d been thinking about electrification and now they could get it for free) and the small collection of branches that came along with the system.