Basically if you work on a preserved line (not sure if you have to work for xxx amount of time) you can buy it for about £17 and it gives you and +1 free travel on most preserved lines ,aslong as it is not a gala, or discount fairs on galas.
but you have to volunteer first to get it , not just be a free loader. and they easily pay its self off.
Some Staff Passes on other Railways can get hefty reductions and some even free tickets at other Railways. Obvious restrictions been at Galas and so forth. e.g. ELR Staff can get a complimentary ticket on GCR except at Special Events.
It depends on your railway and it's relations with other lines. Contact your local TTI for details.