The motor circuit is designed to allow electrical current to flow through the underframe.
From the en.wikipedia.org
Structural strength comes from the underframe.
From the en.wikipedia.org
The company is willing to redesign the already-delivered 30 locomotives of the original order, such that the underframe is strengthened and the weight reduced to less than 140 tons.
From the en.wikipedia.org
More examples
Skeleton: the internal supporting structure that gives an artifact its shape; "the building has a steel skeleton"
The underbody backbone of a car, both for unitary or chassis designs
The component of a carriage or wagon comprising the main structural part which carries the carriage body. ...
The term used to refer to the entire structural framework of the car below the floor, including the center sill, side and end sills, bolsters, cross members, stringers and other attached components.