I don't do off-track, know little of drainage and I've never been an asset engineer but I may be able to give you some idea.
I think an Asset Engineer will be working in the Route Asset Manager's team, who 'own' the asset (I haven't fully got my head round the latest reorg but I think this is still true). You probably won't be running day to day maintenance (Maintenance Engineer) or renewals (Project Engineer) but you will be involved in decisions about when maintenance is no longer cost effective/feasible and a renewals project should be funded by the asset owner. You'll be making sure that both maintenance and renewals are compliant with standards and doing what they should be, and helping to form policy, standards and best practice for the off-track discipline.
As far as questions go, obviously at some point they'll ask something about drains which you should be able to answer given your experience. Beyond that, it will likely start with 'what is NRs safety vision', 'what are the NR life saving rules', safety stuff; they're hot on it. From there it will be STAR type questions; tell us about a time you made a mistake, a time you had to work under pressure, that sort of thing.
Best of luck