The image below was taken inside the first RAC combi two weeks ago and the sheer volume of the space is clear. The bulge on both sides is where the rear passenger door is located on the full passenger version. The cargo bay comes with independent fire suppression. The orange box to the right is a weight used to provide ballast on test flights.
RAC is a part of the Japan Airlines Group and is also the launch customer of the Q400 cargo-combi variant announced at the Farnborough Airshow in July 2014. All five aircraft ordered by RAC will be delivered in combi configuration. RAC has been a Bombardier customer since 1997.