My favorite locomotive would be the Genesis Series P42DC. I like this locomotive for it's sleek and streamlined look, it's engine sounds, and the horn's most of all. While most of the P42DC's on Amtrak are fairly okay in my book, I particularly like the way the Amtrak P42DC's upgraded with GE ITCS(Intermittent Train Control System) look with that added 'box' on the top of the cab above the conductor's side. I think it makes the P42DC look kinda cool, but it's good to know that it serves an important role in safety.

Notice the box added to this P42DC for the higher-speed corridors.