Just learn and memorise as much as you can. You can never tell them too much, but you could tell them too little and sound disinterested.
I had an interview with Northern earlier this week and told them the amount of trains they ran, the number of stations they called at and and the number managed by them, the geographical range of their services, the amount of services they ran per week, the timespan of the franchise etc. It also shows you have a good memory and retain information which is part of the required skill set anyway. I didn’t go into detail about the trains other than the classes of the new trains coming into service soon. I’m not a spotter and didn’t want to sound like one, no idea if I passed the interview yet but I know I did ok on that question!