I'd recommend going through the archives of this forum, the Railroad.net Alco forum, and the World Diesel Locomotive
http://groups.yahoo.com/group/World-Diesel-Loco/
site: this question has been EXTENSIVELY (and, I'm afraid, occasionally acrimoniously) discussed.
As I remember the conclusions of the discussion...
(i) GE has sold the rights to the Alco engine, for most of the world, to (the corporate successor of) Fairbanks-Morse,
(ii) The one country where the 251 seems likely to have a significant future as a locomotive prime mover is India, and GE has kept the rights to the 251 there: there is a newish Indian locomotive with an updated 251 incorporating some of the technology GE has developed for the FDL,
(iii) The FDL (with its cast frame) may actually be a betterr design than the 251 at the sort of power ratings used in modern locomotives (which is perfectly consistent with the 251 having been as good as or better than the FDL in the pre-1980 period of the two design's evolution),
(iv) Even if (iii) isn't so, the 251 isn't ENOUGH better than the FDL to make it reasonable for GE to walk away from all their work in developing and improvin the FDL to start over with the 251, a design similar in age, size, and output to the FDL but which hasn't had anywhere near as much development work done on it in the past quarter century, and
(v) GE's engine gurus seem to have concluded that the FDL -- and by extension a design like the 251 with the same cylinder size -- has about reached the end of its development potential, and the time has come to go for something new: the GEVO.
But do try to find the earlier discussions in the archives: they were very interesting!
