underframe

Etymology

under- + frame

noun

1. The supporting structure of a piece of furniture, a vehicle, etc.
   Nos. 5300/1, which were finished in January, are the Southern's first "4-EPB" units to the standard B.R. design on 63 ft. 5 in. underframes, as against the S.R. 62 ft.; …. 1960 March, “Talking of Trains: S.R. multiple-unit news”, in Trains Illustrated, page 125
   The Liner train wagon is a simple underframe on bogies, with coned location points that engage recesses in the container bases. 1964 April, G. Freeman Allen, “The BRB shows traders the Liner train prototypes”, in Modern Railways, page 265