Thanks for clarifying the Bachmann association. Right country, wrong company.
As far Arnolds line of products I believe that they are both, reusing (and improving) the old Arnold tooling and producing new models.
Here is some info from reynaulds.com:
Arnold, the worldwide known brand in the N-scale, is back in the market with a selected range of new products. Many of the Arnold range of American outline locomotives will now feature NEM couplings and sprung buffers, as well as being pre-fitted with a socket for a DCC decoder.and from tee-usa.com:
Hornby International has recently acquired the renowned Arnold hobby name and model moulds to once again re-launch these high-end N scale 1:160 European trains. With over half a century tradition of fine model train manufacturing, this great line of products is now technically improved with superior motors, LED lights, digital interfaces and much more. Models are factory-fitted with all detail parts and packaged in superior protective shells and outer boxes. These models are of very high quality with the finest detailing and running characteristics.I don't know the full history of the original Arnold but I do recall that they acquired some of Rivarossi's tooling back in the early 90s. I have some Orient Express cars from that time period using what clearly is modernized Rivarossi molds with the Arnold name re-engraved in the floor. As for modifications, Arnold converted them from truck-mounted couplers to body mounted Rapidos on close-coupling swing arms (similar to Kinematic by Kato). There were few other mold changes, but that was the largest one.
So I don't know if at that time Rivarossi was already bankrupt and Arnold picked up some of their N scale tooling, or was that just some sort of partnership. But I think this this was way before Hornby buyout.