Remarkable how these mushrooms are thriving in the timber of this retaining wall. This thickness of timber is generally referred to as a 'Railway sleeper', which our American cousins would call a 'Railroad tie'. They are commonly used in landscaping projects.

 The wood will undoubtably have been tanalised i.e. pressure treated so that chemical preservatives penetrate to the centre of the wood. That is to prevent decay, mould, fungi etc. Nonetheless these fungi are flourishing. 

 I've a notion they may be an edible species BUT many wild mushrooms are toxic. I'm not going to address the subject of foraging for mushrooms in this diary because my ID skills are too limited. I once went on a one day course about edible mushrooms taught by two microbiologists. They seemed to relish describing how horribly you might die from the non-edible ones!

 Furthermore mushrooms are highly absorbent of toxins from their surroundings e.g. heavy metals on contaminated land. Indeed, there is some intriguing research that fungi could be used to decontaminate polluted ground. The mushrooms seen here seem to be resisting and perhaps processing the chemicals in the timber.