Hi Gerry,
The instructions supplied with each pack specify which 'base' pack is required, together with the sounds pack.
The trouble is that I didn't read the instructions properly!
The RDG pack specifies "NAVS_Hopper_100t_Triple_Type1_BASE". Although not indicated, that IS the .zip file name, but if that is put into the main search box, the only download selected is the Paint Kit, which, admittedly, does refer to the required .zip file. The title of the required base pack is : NAVS Z-Series 100-Ton Triple Hopper Type 1 - no mention of 'Base'. This also applies to the Quad set, where its title includes "Foobie Kit", whatever that means!!
Cheers,
Ged