Conservative braking curves for high Emergency Brake Confidence Level (EBCL)
Driven by the trend towards urban living, the number of megacities is growing. According to the latest UN World Urbanization Prospects study, the proportion of the global population living in cities is set to rise from approximately 55% in 2018 to around two thirds in 2050 . Given a parallel increase in rail passenger numbers, this trend is set to pose major challenges, especially for the mass transit sector. One key approach to solving this problem is to optimize utilization of the existing rail infrastructure.
By way of background, we need to consider the regulations that currently define the limits of capacity utilization. At present, the track is divided into fixed blocks that may only be occupied by one train at a time, in order to prevent collisions. Moving blocks will only be possible in the future once ETCS Level 3 and CBTC (Communications-Based Train Control) have been implemented. Until now, extremely conservative underlying braking curves have been used to provide maximum safety, the aim being to ensure that trains come safely to a halt before reaching the danger point, even in adverse conditions.
The key parameters include the Emergency Brake Deceleration (EBD) and Service Brake Deceleration (SBD) curves and the particular characteristics of the vehicle or consist in question. There is some variation in the characteristic values of individual braking system components, such as cylinder pressure, caliper efficiency and friction parameters. Consequently, it is vital to know the probability of the train coming to a halt within the calculated braking distance on a dry track. This is described by the different Emergency Brake Confidence Levels (EBCLs). The higher the EBCL, the lower the probability of the train overshooting the specified braking distance on a dry track.
The EBCL probabilities are used to individually determine the safety level deemed necessary for a particular infrastructure, and can have values ranging from 0.5 to 10−9. A value of 0.5 means that the train will stop within the defined braking distance 50% of the time on a dry track. In Germany, the prescribed value for ETCS Level 2 is 10-7.
In poor adhesion conditions, e.g. on wet or dirty tracks, the sole basis for adjusting/increasing the safety margin of the chosen EBCL level is an additional parameter referred to as Kwet.
A control system that is largely able to compensate for the braking system tolerances can increase braking distance precision on a dry track. Add to this the ability to improve track conditions thanks to an accountable train-wide sanding system, and it becomes possible to reduce the headway between two trains without compromising safety (Figure 1 and Figure 2).