AMTK 90278 is a non-powered cab unit (NPCU). It was originally built in 1977 as AMTK 278, an EMD F40PH.
NPCU's are used on regional trains such as the Amtrak Cascades which goes between two points and aren't turned at either destination. One end of the train has a powered locomotive and the other has a NPCU which allows the engineer to operate from the front of the train. With the engine gone, they are often also used as baggage storage also, which is why there's a large door on each side.
90278 was seen here in Bernalillo, NM on February 23, 2018 on the eastbound Southwest Chief. It was probably deadheading to Beech Grove, IN for servicing or perhaps reassignment.