Anyone who would have reason to be on the track or within the railway land alongside the track bed. This would include track gangs, shunters, drivers, guards and depot staff. The main differences over time have been additional colours and the addition of reflective strips. You could also a hard hats for track gangs, shunters and depot staff.
The design of hi vis clothing has changed also over this period, back in early 90s the waistcoats would have had rubber buttons and a couple of reflective strips and a reflective panel with BR logo on the rear. Now the waistcoats have Velcro fasteners and multiple reflective strips, they also carry many different logos for track maintenance firms. You will also notice similar changes to the other clothing that is worn such as weather proof jackets and leggings.
Hope this is of some help.