RPMs manage RP Assistants and Inspectors, so have a look at their roles, and the relevant bits of law (Police & Criminal Evidence Act, Railway Bylaws, Regulation of the Railways Act, The Railways (Penalty Fares) Act etc). There will probably be the usual people management questions around leadership, problem solving etc, and maybe a couple of technical RP questions around relevant laws and procedures (ATOC have a summary of the basics of the procedures called Arrangements for travel ticket irregularities which you can google as its public ally available). Maybe read SWRs RP policy which you can get on their website.