If the available resources are not sufficient to maintain lines and tracks at stations according to minimum safety requirements it is from a derailment and safety viewpoint better to close the lines or tracks for operation than trying to keep lines operational in a state where all safety margins are removed.
Accident investigation reports from various countries have shown that many accidents occur due to known infrastructure failures that there might not be resources to repair, or such repair has not been prioritized within available resources. Such conditions increase the risk of freight derailment and if hazardous materials are transported on such lines it might be a public risk. 5.5.3.2 Inspection and maintenance to ensure free clearance gauge
The clearance gauge should be kept free of obstructions when trains are due to arrive. This is a general inspection and maintenance task carried out by all infrastructure managers. Special focus should be given to the flange groove in level crossings. If the flange groove is obstructed by hard solid objects it can cause derailments. Level crossings with rubber elements (Strail) can reduce the risk.
In countries with severe winters snow ice can pack in the flange groove and around the rail during periods of frost during night and thaw during daytime. In particular this can be a risk if free water seeps over the track, for instance in level crossings. The risk is most severe for passenger trains.
5.5.3.3 Ultrasonic Rail Inspection Wagon
The infrastructure managers provide for ultrasonic inspection of the rails by various forms of wagons in order to detect cracks and fractures that can cause rail ruptures. Either the infrastructure manager owns the inspection equipment or the inspection is done by contractors. The ultrasound inspection provides the infrastructure manager with information with regard to the quality of the rails and the need for rail replacements.
The frequency of ultrasonic rail inspections is determined by the infrastructure manager based on the rail age and traffic loads on the actual line accounting for available resources and equipment performance.
5.5.3.4 Track Geometry Measurements
Regular track geometry measurements are carried out by most infrastructure managers. In order to be reliable they should be carried out under dynamic loaded conditions. The track geometry of railway lines is regularly measured by track inspection wagons or trains which provide dynamic loading to the track while doing the measurement. Among the geometric parameters measured are:
Track gauge variations.
Track cant.
Track twist.
Track height variations.
Track lateral position faults.
In addition modern measurement wagons can inspect rail surface conditions in terms of rail wear and various rail surface defects. The completeness of the measurements with respect to track coverage at stations as well as intervals may vary. Frequency is normally dependent upon traffic load and allowable speed limit of track.
The frequency of inspection is based on local conditions and environmental factors, ground stability, line speed and traffic loads accounting for available resources and equipment performance. Normal frequencies can be 2 to 6 times a year with increased frequency for lines with more traffic and higher allowable speed.
5.5.3.5 Track Twist Intervention Limits
Excessive track twist is among the most frequent derailment causes often in combination with other causes such as skew loading, wagon frame twist and low speed in narrow curve with high cant etc. In many cases where track twist is a major factor leading to derailment the actual track twist exceeds allowable twist limits, and in some cases the situation has also been known to those responsible for track maintenance.
Track twist requirements must be looked at in combination with requirements and limitations for rolling stock flexural stiffness. The ORE B55 RP8 document has analysed the conditions for derailment. Ref./8/.
The final draft TSI for Conventional Rail Infrastructure specifies safety limits (or immediate action limits) for track twist as follows:
―All TSI Categories of Line
(1) The immediate action limit for track twist as an isolated defect is given as a zero to peak value. Track twist is defined as the algebraic difference between two cross levels taken at a defined distance apart, usually expressed as a gradient between the two points at which the cross level is measured. The cross level is measured at the nominal centres of the rail heads. (2) The track twist limit is a function of the measurement base applied (l) according to the formula:
Limit twist = (20/l + 3)
(a) where l is the measurement base (in m), with 1.3 m l 20 m, (b) with a maximum value of 7 mm/m.
(3) The Infrastructure Manager shall set out in the maintenance plan the basis on which it will measure the track in order to check compliance with this requirement. The basis of
measurement shall include at least one measurement base between 2 and 5 m.
TSI Categories of Line IV-F, IV-M, V-F, V-M, VI-F, VI-M, VII-F and VII-M
(4) If the radius of horizontal curve is less than 420 m and cant D > (R – 100)/2, track twist shall be limited according to the formula: Limit twist = (20/l + 1.5), with a maximum value between 6 mm/m and 3 mm/m depending on the twist base length as shown in Figure 4.
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The above limits specified in the TSI are safety limits that require immediate traffic shut down. According to recent accident investigation reports several derailments have occurred due to track twist in tracks within the safety limit specified above.
The TSI specifies that intervention limits shall be developed by infrastructure managers or national safety authorities (NSA). Today‘s intervention and safety limits for track twist varies somewhat between different countries within EU.
An existing measure adopted by some infrastructure managers has been to impose more stringent limits for these parameters which suggest a more widespread adoption of harmonised limits may be beneficial. The reason for this is that rolling stock meeting the TSI for freight wagons is interoperable through the European Union and hence criteria for track maintenance activities should be harmonized in order to be able to maintain a high level of safety against derailment due to track twist. The intervention and safety limits should be viewed in relation to the lubrication status of the track.
Further, one should make sure that the developed criteria can handle allowable skew loading conditions of wagons with a certain margin.
5.5.3.6 Immediate Action Limit for Variation of Track Gauge
The immediate action limits for variation of track gauge are set out in the final draft TSI for Conventional rail.
Speed [km/h] Dimensions [mm] - Nominal track gauge to peak value Minimum track gauge Maximum track gauge
V 80 -9 +35
80 < V 120 -9 +35
120 < V 160 -8 +35
160 < V 200 -7 +28
The above immediate action limit is significantly less rigorous than today‘s action limit for many countries as for instance GB /20/ and Norway /35/. A review of the limits may be warranted if there is a strategy to reduce derailment frequencies. The argument for harmonised limits is as for 5.5.3.5.
5.5.3.7 Immediate Action Limit for Variation in Cant and Excessive Cant
Action limits for variation in cant relative to design cant is specified in the final draft TSI for Conventional Rail Infrastructure.
TSI Categories of Line IV-F, IV-M, V-F, V-M, VI-F, VI-M, VII-F and VII-M (Requirements for passenger lines (P-lines) are excluded as they are not open for freight traffic.)
(1) The in service cant shall be maintained within +/- 20 mm of the design cant, but the maximum cant permitted in service is 170 mm.
Additional to the above some countries, such as Norway and Switzerland, have general limitations of allowable excessive cant, specifically at locations where trains are expected to stop at a signal or drive slowly /33/ & /34/. This requirement is of special importance at locations with narrow curves where trains may have to stop in front of signals and where there also is high track twist when leaving out of transition curves.
5.5.3.8 Immediate Action Limitation for Track Height Variation
Among others, the railways of Norway and Britain have intervention limits for variation in track height. The intervention limits specified in Britain and Norway is relatively consistent, but with some minor variations. Variations in track height and cyclic tops may cause derailment, in particular if there are cyclic variations. A report issued in January 2006 as a result of a research work financed by Rail Safety & Standards Boards identified height variations and cyclic tops to be one of the most frequent high speed derailment causes /21/.
A measure could be that the Final draft TSI for Conventional Rail infrastructure is modified to include quantitative limitations on height faults. An interoperable rolling stock fleet will benefit from harmonised track intervention and safety limits.
5.5.4 Rolling Stock Applied Technical Measures