In point of fact, ALCO did offer a C-428, and evidence of this actually is pretty easy to find. Find / pick up a copy of ALCO manual TP-447, which is the operating manual for Century series units with DC transmission, and you'll find the type listed on the specifications page (with concurrent Century series identification as model 428 and specification DL-428.)
The ALCO C-855 was essentially two sets of the equipment for the DL-428 mounted on one chassis and had exactly the same characteristics as those planned for potential DL-428 units, which also are as you named; 16-251C (1050 RPM), GT-598 generator. I should note that, as on the GE U28B, standard gearing for the unbuilt C-428 would have been 81/22 with a top speed of 74 MPH.
Evidence of a designed but not built C-620 is harder to find. Years ago, I went with my brother to a railroadiana show in Columbus, and during a couple hours of searching found an ALCO sales catalog. The price was way too high, but I did flip through it. I did in fact find complete specifications and drawings for an offered but not built C-620, and made a point of pulling my brother over and saying "hey, check this out and make a note of it" and naturally he did. The unit (interestingly) appeared somewhat like that photoshopped drawing, in that it had a relatively long short hood and much less space between the cab and the central air system intake. Twelve cylinder engine, and GT-586 generator.
I'd like to note for those who are wondering that I did ensure while having the book in my hands that this was NOT an export locomotive.
Had this sales catalog NOT been around two hundred dollars, I'd have it now. As it is, though, I do also recall that particular listing being the only thing that surprised me in the couple of minutes I took paging through it. Luckily, the seller didn't say "hey, you gonna buy that or what?" because I already had handed him something else off his table and said "hey, can you put this back there for me while I keep looking?"
Finally, some comment was made as to slipperiness of units such as would have resulted had any C-428's been built, and I would bring up the fact that GE built a good number of U28B units in the year long production run that this model had, and did have mixed results as regards adhesion. Experience with the U25B and U28B led GE to develop further improvements in slip detection which appeared in the production U30B units that followed. (These included the power tie detection circuit, effective at very low speeds, and two different circuits which detected low-speed synchronous slip and high-speed synchronous slip and spin; the previously applied axle-alternator system was retained with these other additions, and the slip suppression braking {SSBV} was fitted as the customer dictated.)
-Will Davis
Last edited by Typewriters on Thu Sep 07, 2006 9:09 pm, edited 1 time in total.