Michael
I think you answered your own question. Time for rebuild will be similar. However, it was stated that 476 would cost more. So you choose the more "painful" one and get it out of the way. I know in my line of work, this is the approach we like to always take. If possible, do the hardest and most difficult first. Get it out of the way. When you have budgets and timelines, it is usually better to tackle the difficult first, it gives you time to plan and adjust if needed. If you go the opposite, the tough jobs usually bite you and this will cause a multitude of issues, like liquidated damages. Granted, as they are doing the work themselves, they won't have this issue. But, if they are projecting growth over the next ten years, then doing the 476 at the end could cause a real problem.
If we are just wondering, perhaps the restoration of 476 was being looked at, even with the 483 deal. But since that locomotive would have needed even more for restoration, it would have been first with the 476 following her after its return to service. There is also the fact that the 476 coming back will allow a different type of fan fair than the 478 would have. Now if 478 is down for say 8 years, then the fan fair for her return will be greater too. I would also surmise, that if it was needed it could be turned around more quickly, if need be, and would be serviceable to get them over the hump. But they would like the 18 month window better. I know the circumstance's are not the same, but the 482 was brought back in what, over the winter, when she arrived after the 497 trade? My guess is that they know exactly what 478 needs. With the 476, they are pretty sure bet as she has been sitting, there may be some hidden issues that have not made the rebuild list yet. When 476 was put on display, is was with the intent that she could be restored IF needed. 478 is being put on display with a timeline of when she WILL be restored to service.
The DSNG explanation makes complete sense to me, and it will allow a stuffed locomotive to comeback to work. Wa Hoooo!!!
Phil