Retention of wooden sleepers

Could be over places like bridges and structures where wooden sleepers are lighter

Also anywhere with trainstops fitted have to have wooden sleepers and more also around points

Old fixings its abit difficult to make holes into a concrete sleeper. Each one is different believe it or not especially on curves its all done on site and with a manual drill nothing petrol or electric driven

Thanks for the explanations, I'll have to look a bit more closely!

It may be to allow the track to expand in heat and contract in cold wooden sleepers have a little flexibility compared to solid concrete.

I commonly see wooden sleepers used at expansion joints where a section of bullhead rail is bolted flat to a set of sleepers to hold them together.

You missed the gist here methinks! The topic was about wooden sleepers at flat bottom rail expansion joints. This is a cheap version of the F/B expansion joint where the wooden sleepers support the expansion joints and are tied together with two short lengths of B/H rail laid on their side to keep the road together end to end. F/B expansion joints laid on the usual pads on standard concrete sleepers will not perform as required unless the sleepers are tied together i.e. in severely cold weather the track would simply shrink away from the expansion joints with nothing to stop the contraction.
It is possible to see expansion joints on concrete sleepers outside LUL but the sleepers are made with holes in the four foot to take ties. There are also different types of expansion joints more commonly known as expansion switches which look similar to stock and switch rail pairs and which are mounted in a form of F/B slide chair, this being the expensive type as it requires more components and usually seen on high speed rail lines.

I don't "know" that that is the reason why but I do know that it appears to be the case and is expedient. Perhaps cost is a factor but wooden sleepers are readily available and are flexible in that all sorts of equipment can be mounted to them as they are easily drilled with a P-Way auger. On the other hand concrete sleepers have to be manufactured with the holes pre-drilled for specific applications. Wooden sleepers are tried and tested and have stood the test of time for around 200 years from the earliest forms of railway seeing off other types made of metals long ago. Concrete itself is not new but I suspect it is cheaper to manufacture sleepers from it than to wait years for Australian Jarrah to grow. Of course traditional Jarrah sleepers are rarer these days and modern sleepers are made from pine which is not so long lasting being a softer wood and quicker to rot despite the preservative. Concrete sleepers and long welded flat bottom rail give a smoother ride and better facilitate ballast packing by machine and simple track is quicker to lay but I suspect there are more complications than not laying points and crossings on concrete sleepers even if they are made specifically for the purpose so wooden sleepers will continue to be used in such areas for cheapness and efficiency of installation.

It would also be due to the fact that very rarely are two sets of points the same making it so much easier to use wood that a custom concrete sleeper for each part of the point. Also the shape of a concrete sleeper is sort of U shaped, rather than completely flat.