Well there is one difference, electric motors seem to last much longer than diesel loco's. I'm not sure if that's something that's unique to Amtrak and how they handle things, or if there are real differences that create the longevity issues.Eh, for American passenger locomotives, there really isn't a cost difference (and electric locomotives are cheaper to maintain). Metrolink's EMD F125s will be about 6.47 million, the ACS-64 is $6.65 million. But honestly, locomotives shouldn't be purchased at all and the ACS-64 purchase was an absolutely terrible decision; EMUs outperform in every way that matters.The problem with electrification is two-fold:
(1) It's a pain to maintain. Let's face it, hundreds of miles of overhead wires? Not cheap.
(2) You have to have dedicated locomotives and so forth, which aren't cheap (and which are more expensive to acquire than standard diesels).
As for Amtrak buying the ACS-64, it was the only decision that could be made. They need to start replacing the AEM-7's which are nearing the end of their useful lives, and the HHP-8's are not only not plentiful enough to fill the gap but they also have their own failure issues.
And Amtrak barely scrapped together enough money to buy new locos. Buying 400 EMU's was out of the equation, there was no way that they could find that money and no way to convince a fickle Congress that they should replace every coach in the inventory all at once.