If I understood correctly back in October when I last used R&F, you can only go as far as any given railway route goes. For instance, my gf and I were booked FRA - Essen, so we were allowed to make a stopover in Düsseldorf outbound and in Cologne inbound, as they were en route. However, we couldn't have gone to Munich and then to Essen, for example, as Munich is not en route to our final destination. We would have had to buy a ticket for the Munich - Essen leg.

Whether the ticket agent onboard would have bothered with our tickets sufficiently to figure that out is another question.