Trex is not structurally stable over the spans between the factory cross sill locations. It will weather well and resist rot, however you do not want it to sag under loads that may be introduced when hauling items.
The original wood was southern yellow pine, however the harvested wood from the forties, fifties and sixties is from the "old growth" forests, whereas a lot of what is available today is from "new growth" forests. The wood from old growth forests has better structural and decay resistance characteristics. In any event, there are many options available. White oak is preferable to red oak from a stability standpoint, however red oak is more readily available, thus more economical. Some prefer red oak over white due to the richness of the color and the interesting grain patterns. If you are looking for original, go with southern yellow pine. If you prefer a more showy look, red or white oak may be your choice.
Good luck with your wood plank replacement.
