Page2

The vitiation of the conditions cannot lead to loss of considerations of safety. The solution to this is to provide signals which will satisfy conditions of safety as well as ensure that the operational requirement. Requirement of safety under this condition can simply be met by ensuring that the signal in rear which is at a distance more than the  braking distance from the signal showing either stop or the setting of route which is a turn out at yellow. This solution however can reduce the speed of  trains too early as the signal in rear may be far from the signal at which the train has to stop or take a turn out. Hence the introduction of a fourth aspect helps in the matter. The fourth signal is not a new color of the signal but is the use of two yellow signals on the same post and is referred to as double yellow. The double yellow signal when used can be used till the braking distance is available and can be repeated over several signals leading to the signal immediately in rear at which the train has to stop actually. The signal immediately in rear should display a yellow aspect. While here again the train can be made to slow down unnecessarily but practically this is a good enough solution for most conditions of manual train running. In figure 3 the use of signal S3 about 1Km in rear of the diversion takes care of this need.

Classification of signals

Main Line Signals at a  station can be classified into the following:

a) A signal which controls entry into  and directs it to specified line of  a station yard which is in many cases referred to as Home signal and can be generally called the first stop signal of  a station  Signal S5 is a Home Signal in Figure 3.

b) A signal which allows departure from specified lines of a station yard which is in many cases referred to as Starter Signal. In Figure 3 Signals S7 and S9 are starter signals.

c) A signal which  allows a train to leave a station yard  which in many cases referred to as Advance Starter Signal and can be generally called the last stop signal of a station. Refer to Fig 4 where Signal S11 is a Advance Starter Signal.

d) A signal which is at adequate braking distance from the from the first stop signal and often called the Distant Signal and is a  approach repeater signal. In figure 3 Signal S3 is the distant signal.