Train Protection Warning System OSS Fitment

Train Protection Warning System OSS Fitment

Presentation Train Protection Warning System OSS Fitment

Train Protection Warning System OSS Fitment

Whether OSS is required at Signal?

There are 6 main conditions under which an OSS is not required at a signal:

1. The Safe Overrun Distance is adequate.

2. The signal is a platform starter at a terminal or turn round platform with speeds generally below 25 mph.


3. There are points in the Overlap of the signal beyond and Warning controls are in place.

4. Certain signals (other than Home) in Absolute Block, Electric Token Block & Tokenless Block signalled areas.

5. Stop Boards in RETB signalled areas.

6. Stop Boards in NST and NSTR signalled areas.


Adequate Safe Overrun Distance

Safe Overrun Distance Required


OSS Loop Positioning Calculations For Signals



OSS Loop Positioning Calculations For Signals



Calculation of Trigger loop position

7.4.3 OSS Trigger Loop nominal position

The nominal distance from the signal to the leading edge of the OSS Trigger Loop (in the direction of travel) shall be determined from Table 5


Adjustment to Gradient


The nominal position is adjusted by adding / subtracting a figure, dependent on the steepest falling gradient (or shallowest rising gradient) within 400 metres in rear of the signal. In the most extreme case, the nominal position can be extended by 75 metres, to 425 metres in rear of the signal.

Adjustment to Gradient

Table6- Gradient Correction Distance

Calculation of Arming loop position


Calculation of Arming loop position


Final Adjustments


There may be constraints due to the non-permitted loop positions already discussed, which prevent the arming or trigger loops going in the calculated positions. In this case, the loops may be moved by a maximum of 25 metres in either direction, before settling on a final position.

Note: if this adjustment is by more than 12 metres, the Trigger loop position will fall into a new 25 metre band in Step 4, and so the loop separation will need to be re-calculated too.


TPWS+ Requirements and Positioning

  • To improve the level of protection at certain signals, TPWS+ can be used which consists of one additional OSS to provide train protection for trains travelling at speeds in excess of 75mph, up to a nominal 100mph (see Figure 11). The additional OSS is referred to as OSS+.





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