With the AEM7s soon to be gone, Amtrak's oldest locomotives in revenue passenger service would be Dash 8-32BWHs, which were built in 1991, followed by the P40DCs (1993), the P32AC-DMs and HHP8s (1996), and the F59PHIs (1998).
Some people might also count the NPCUs even though those are unpowered. I remember hearing that Amtrak considers them "locomotives" for the purpose of their rule books. Those date back to the mid to late '70s.
MEC407
Moderator:
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives