I recall a thread on the Atlas forum started by the late Jim Weaver involving their first locomotive, the SW8/SW9. It was a guessing game of sorts. Basically, the decision was based on eras covered -- transition to the present, roads that used it -- a lot, flexibility -- yard and local freight, variants -- SW900. Very successful model.
From a business standpoint, particularly with the rampant spread of three-rail rivet counting (admit it, guys; you'll feel better), there are factors similar to those Atlas used with the SW. How they can maximize the use of the tooling and production investment with something that will sell. The level of detail being demanded raises production and tooling costs, so that has to be amortized across multiple unit sales. In the past, you could get away with non-prototypical equipment in the 3-rail market, but not so much any more (heritage units aside). So the new offerings seem to be much more selective -- perennial favorites (F-units, E-units, certain steam, popular diesels). I haven't seen a lot of one-offs (road unique) unless there's some historical significance (S2 Turbine, DD40Ax, AGEIR box cab, Cab-forwards, Big Boys, Challengers, etc.). I'm still hoping for that CF7, but while it's historically significant in its creation, Santa Fe was the only road that had them (ironically, they're all over the place in branch line/industrial service).
