Don't let them know you are a railway nut they tend to shy away from spotters as for cleaning I would do anything if it meant getting a foot hold in a job I wanted , look at it as a learning curve it's all good and you'll be getting paid.
There's no harm In letting them know you are an enthusiast as we have many that work for us as drivers!. The key Is getting across how you can transfer the skills you've gained from being an enthusiast Into the "full time railway"
For example positively you could use that you work with the general public and have had to do the heritage railway's own PTS course and have a good grasp on safety and working to rules and regulations etc etc etc . What you don't want to do Is make out you are that much of an enthusiast that you will be spending as much time taking pictures of the steam train parked on the next platform as you are concentrating on the 2 buzzes from the guard that you have just received therefore compromising safety.
That Is where a lot of enthusiasts fall short which Is a shame as you'd see someone who loves the railway that much as the perfect candidate!........they just need to find out why they want to actually work for the railway, If It's just to spot trains day In and day out It's just got Incident written all over It.