Slightly off topic but as I have seen ballast mentioned on here I will ask... What is the purpose of ballast? I know it's there etc but I have never actually been told why or the purpose of it.
hock:
It is to give you something to put the sleepers in.
It is what it is because experience over the years has taught that of the various substances tried over the years rough angular 2" lumps of hard ignous or heavily metamorphosed rock Are the best answer to providing long lived adjustable support, drainage, and resistance to movement.
Substances used as ballast over the years include.....
Ash mainly on sidings and very minor lines, once in very plentiful supply at any MPD. Needs a lot of small scale work to keep on top of it, and does not like being wet. May also contain acids which are bad for the rails and heavy metals which are bad for the environment. Although when fresh this combination does also act as a weed killer. Ash was xome times used on main lines where there was a need to reduce the weight of the track due to earthworks problems, but this would be at the cost of a much greater manpower input to keep the track in order. At least on derailment (at Bletchley) was caused by this dodge going wrong.
Beach a mixture of sand and gavel similar to what a builder would think of of as being ballast, once used on lines far from hard stone quarries. Disadvantages are that the fines content impeads drainage and encourages weeds. The round stones are not as good as angular for holding top and line, and if the material was marine dredged the salt content can be bad for the rails.
Blast furnace slag mainly used up in the North East as that is where the steel industry was concentrated, the LNER was also short of money so something cheap also suited them. This was a half way decent ballast but did not last as well as hard stone, it is also lighter which is why some ballast hoppers in that part of the country were fitted with raves to increase their capacity. Not used these days but can sometimes be seen on more minor lines.
Tunnel ballast ~ 1 1/4" ballast once used as top stone in dry tunnels where the ease of working with it and lack of need for drainage ability made it worth while to go to the trouble of using a non standard ballast. Still occasionally used on lines where getting a tamper in was very difficult, or sidings up to ten or fifteen years ago. But nowdays no one can be bothered, so the trackmen just have to work with the normal sized stone.
Standard stone ballast 2" lumps of hard stone usually called granite although some of it is other similar looking and performing rocks. Used to be sourced at various quarries scattered round the country a hoppers trip working from the various rail centres. Now much of it is quarried at Glensanda in the Scottish Highlands and brought round to coastal loading points by sea. These are good durable ballasts, the main cause of their break down into fines is tamping. So only tamp track that needs it, or you will soon be needing a ballast cleaner instead.
