Came across this useful piece of info related to railway safety:
What is Neale's Ball token instrument?
Designed by Neale's,an Engineer with the G.I.P Railway(presently Central Railway),it is an electromechanical instrument provided at each station on single line railway sections.It ensures safety in train operations by dispensing tokens that are handed over to train drivers as authority to enter a block section.The tokens are spherical steel,balls,which are issued in such a manner that only one token can be issued for one direction at a time.,after ensuring that the previous train has already cleared...
more... the section and there is no other train between the stations.Each station has one such instrument for each direction,which are electrically connected to similar instruments provided at the adjoining stations on either side.This ensures that only one train can enter the block stations at a time.
Source:Mind Teasers -An A for your Q..a TOI publication.
(Excerpts from 'Open Space' column of TOI)