Here is a picture of an F7B in Carlsbad, New Mexico in 1978.
On a “slug” unit, the prime movers have been removed and replaced with heavy weight, such as blocks of cement. Just the traction motors remain and they get power from another locomotive. They increase the tractive capacity of the accompanying locomotive.
These slugs would have passed thorough Roswell prior to the 1990s. F units slugs were no longer used by the 1990s.