Rich,
If there was a way to get up to the 500+ range of units on this locomotive, It would certainly be a go. There is no bias as to what model Sunset would like to produce or not produce, it just comes down to reservations.
Ten roads purchased the original version. Then there are of course some oddball paint schemes when some went into lease service. The SD60F is probably too much of a tooling change, but the SD60I and SD60M might be possible to increase the number of roads and provide variation for roads that bought both. I'm not aware of many modifications made to the base model, but I'm sure you and others could enlighten me on any road specific mods.
I agree the MTH one is old tooling and that is why mine, which was an auction bargain to start is going to a new purpose. Both MTH and Lionel also came out with their version of the SD60E recently. Not sure, that variation would be viable at this point or not.
