Definition of "underside"

A correspondent suggested that the superior riding qualities of the latter were due to the absence of piston-valve chests above the cylinders, which made it possible to bring the underside of the water-tanks to a lower level than that of the "Rivers", so minimising the unsteadying effect of water surging in the tanks.

1944 May and June, “Notes and News: Tank Engine Stability”, in Railway Magazine, page 185

[...] and the neat reading lamps on the underside of the luggage racks, under passenger control, are an attractive feature of the first class compartments.

1963 March, “The Clacton express electric multiple-units enter service”, in Modern Railways, page 173

The Eagles then hit the woodwork twice in a matter of seconds as Scott Dann headed Jason Puncheon's free-kick against the underside of the bar, before substitute Mile Jedinak also hit the crossbar with a shot on the turn.

2016 January 23, Michael Emons, “Crystal Palace 1–3 Tottenham”, in BBC Sport‎