Speed Restrictions

Author: Daniel Woodland
Co-author: Joss Apps
Conference day 2
Performance Session, reserve paper

In normal operation, a train’s speed must not exceed either the safe limit (before, say, derailment may occur) or the comfort limit (where passengers may be bounced about or otherwise experience unacceptable discomfort). These characteristics can between them be defined as Vmax for the particular train type at any given location.
Since ATP systems are provided as safety systems, they must ensure that unsafe levels of overspeeding do not occur. At the same time, in order to support efficient operation of the railway, they must not be overly restrictive in constraining a driver’s normal (safe) driving behaviour.
If an ATP system is to prevent the train speed from reaching unsafe levels, it must:
• Allow for delays in brake application once an intervention has been made;
• Allow for the system’s own processing delays;
• Allow for worst case acceleration of the train during these delay periods.
It must, therefore, initiate an intervention at a speed well below the actual safe limit.
In addition to the constraints imposed by safety, many operators also require ATP systems to:
• Provide a warning to the driver if he/she is detected as overspeeding;
• Allow the driver some reaction time following a warning before intervention occurs;
If such facilities are provided, the speed at which warnings must be initiated will be even further below the actual safe limit. However, in order to ensure efficient operation, such warnings are generally required to commence only once overspeeding has occurred (i.e. the train is travelling at a speed which exceeds the maximum ‘authorised’ or ‘permissible’ speed).
At this point, the implementation of ATP supervision starts to look like it will have a significant (detrimental) impact on operating speeds if drivers want to avoid intervention. Fortunately, however, the permissible speeds applied to the railway are not set at the maximum safe and comfortable speed for travel (Vmax). In general, whilst a line speed or Permanent Speed Restriction (PSR) will be based on an assessment of the track topography (and condition) and perceived acceptability (safety / comfort of ride) of movement at a given speed, that assessment is made qualitatively, based on (conservative) ‘Engineering Judgement’ and it will be formally defined as a nice rounded (down) number that can be easily displayed, read and understood from trackside signage and route descriptions. Thus, the PSR is in-fact significantly below the Vmax. In degraded conditions, these may be superseded by a lower Temporary Speed Restriction (TSR) or Emergency Speed Restriction (ESR) in order to protect against concerns that operation at the PSR is no longer safe, pending remedial works – but the basis of calculation for a TSR / ESR is similarly qualitative.
On the Mainline Railway maintained by Network Rail there are several differential PSRs, probably the most significant is that for Sprinter Trains (‘track friendly’ trains, allowed to operate at higher PSR compared to non-Sprinters passenger rolling stock and freight). Differential PSRs recognise that there are differences between train types.