I think it's too soon to tell whether the overall concept has been a true success, but it's worth noting that GE has built over 1,000 ES44C4s, and is now busily building the ET44C4 Tier 4 version. BNSF's C4 order is on par with UP's famous 1000-unit order of SD70Ms. C4 production is also on par with the total number of SD60s EMD built, and exceeds the number of GP38s they built, so I'd say the GE C4 is a success even if they never build another one from this point forward. EMD hasn't had nearly the same success with their P4, but perhaps that will change once they get their Tier 4 production going. We'll have to wait and see.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives