Martian Metals made the figures. The 15mm Traveller figures are still around on discount sites, the same scale as the Grav Ball figs. Not sure when or if Martian Metals is still in business or which company ended up with the molds. I got the complete set of Traveller figs in 1981.
The model railway N gauge isn't the same scale. 15mm (the Grav Ball figs) is a popular miniature gaming scale and there are numerous Sci-Fi figures out there that could be replacements.