underfloor

adj

1. (not-comparable)Located beneath the floor.

   “The ride quality is pure HST - you have a smoother ride than on any underfloor DMU, aided by less intrusive noise. Sitting at the end of the coaches (and thus over the bogies) is obviously that bit bouncier, but in the centre of the trailer cars it's lovely - and much better than so many new trains such as Voyagers or Class 180s... in fact, anything with those dreaded underfloor engines.”

noun

1. A layer of flooring beneath the main floor surface.