There are some GWR bench marks still in hedges not far from my house. They are larger about 8in diam, 3-4in thick cast iron and forged onto the top of lengths of rail track, driven into the ground. They are in hedgerows about 100yds away from the railway fence. same sort of thing I expect. As a child I grew up and within 1/2 a mile of the railway and explored fields close to the line for several miles and never saw these. Why these ones have stayed in place I have no idea.