<br> Being the simplest, it has drawbacks, not the least of which is that it is exposed to anybody or any thing which might come into contact with it. Most varieties of top contact shoes simply dangle from a beam suspended between the axleboxes of the bogie. The shoe is suspended from an insulated beam hung between the axleboxes. Whatever the origin, shoe has caught to today. The commonest reason is when a shoe breaks off and its connecting result in the electrical gear on the practice needs to be secured safely. The 3rd rail system is common world wide but the 4th rail is uncommon. AC techniques always use overhead wires, DC can use either an overhead wire or a 3rd rail
