That’s the chassis for a very common type of mine, quarry and factory narrow-gauge railway wagon, so would have been conveniently readily available for that training-range use.
In mine mode it often carried a side-tipping hopper of Vee-profile, tipped by a curved angle-steel rivetted to the tub, and rolling on the horizontal cross-bar at each end. The central peg still in place at the near end located the hopper in upright position.
The axle-boxes carry roller-bearings, and the suspension is limited vertical movement against rubber pads.
The “arrow-head” at the near end is the top of a coupling-pin, I think to take a chain-link.
Considering it’s been buried in sand and salt-water for so long, it’s in remarkably good condition!