deano, if you're referring to the Amtrak diesel locomotive that it tows nowadays, it's mainly for HEP, dynamic braking, and to keep Amtrak's fears of a breakdown at bay. 4449 ran without a diesel locomotive on this trip, and the 2004 Montana trip (and numerous others, including excursions in 2009) because it operated under an insurance policy that had nothing to do with Amtrak. Those trips also had an in-consist generator car that supplied power to the train (just like what Amtrak diesels do nowadays). As far as I know, there are no generator cars that are Amtrak-certified, and there's practically no reason to bring one up to standards since Amtrak diesels already provide HEP.

4449's partnership with Amtrak is a huge part of how it's been able to continue running mainline excursions starting in 2005, and part of that includes having "up-to-date" equipment *with* HEP capabilities, so Amtrak diesel locomotives are here to stay. There have been discussions about modifying the Auxiliary Tender to include a HEP generator, but funds are more important elsewhere. If someone would like to chip in $$$ towards modifying and installing an HEP generator in the Auxiliary Tender, the 4449 volunteers will be happy to talk to you! ;)