I thought this was a copy of the Atlas or Model Power 40' gon. Both of which would take a lot less effort to convert. The glue used in these and the Bachmann gons with coal load would melt and deform the interior.
The High Speed "coal car" is longer than a 40' gondola and has 12 side panels (vs. 8 panels on either the Atlas or Bachmann 40' gons)
It's also not as long as a 50' gon, but I haven't had a chance to measure one to know what to call it...
Regarding the coal load, both the coal load and the underframe are held together by way of a single screw on the bottom of the car.
The good news is that this "sandwich" assembly negates the need for any messy glue to hold either of them in place, but the bad news is that this leaves a gaping hole in the middle of the gondola floor after the coal load and underframe have been tossed in the nearest dumpster.
CJM