I had to look this up. From Wikipedia:
Amtrak conversion to cab cars
In an effort to extend the life of its existing fleet until the arrival of the
Airo fleet, Amtrak began testing a converted HHP-8 locomotive as a cab car in 2023. These converted units are designated as HHP-8C.
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The project initially aimed to convert two locomotives for expanded
Ethan Allen Express service in 2022. While the first conversion (originally numbered 691, initially renumbered 90691 and later 9750) faced initial setbacks, it was ultimately successful. As of July 2024, Amtrak is considering expanding the program to include up to eight total conversions.
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