The low adhesion problem due to leaf contamination in the wheel/rail contact: Bonding and low adhesion mechanisms

Abstract

Autumn leaves often cause low adhesion problems for train operations, leading to station overruns and signals passed at danger (SPADS). The aim of this paper was to review operational data and research methods to assess the current understanding of the problem and formulate hypotheses for the causes. Incident analysis showed the relatively high possibility of incidents between the hours of 05:00 -10:00 and 20:00 -24:00, suggesting the dew effect was important. This result corresponds to the knowledge that wet leaves in the contact area produce very low friction coefficients, below 0.1. Current mitigation methods, such as sanding, seem inadequate to remove the leaf films completely. To explain the bonding mechanism between the leaf film and the rail, a laboratory-based model and a field-based model were developed based on previous studies. Moreover, key parameters for a strong bond formation were identified, which are iron oxide, temperature, pressure and leaf material. The research gaps were identified by a paper grading method, and several hypotheses for bonding mechanisms and low adhesion mechanisms were proposed, such as sub-or super critical water and pectin gel.

The composition and friction-reducing properties of leaf layers

“…The resulting suspension was filtered to remove the large debris. This method is in accordance with Ishizaka et al [5], however for this study all BLE suspensions are further filtered down to 0.49µm resulting in a clear brown solution with a pH of 4.8.…”

“…However, rail travel only accounts for 10% of the total passenger kilometres travelled [3]. Dissatisfaction in rail services is driven primarily by delays [4], with seasonal delays caused by leaf fall being notoriously problematic [5]. Field observations and laboratory tests indicate that fallen leaves leaves are crushed in the wheelrail contact leaving a black layer [6,7].…”

“…In this study brown leaf extract (BLE) was prepared and tested using a variety of techniques. The extract was prepared in accordance with the method given in [5], which will be related in brief here. Brown sycamore leaves which had naturally fallen were collected during the autumn of 2018, these were milled into a mulch with an average particle size of 5mm, and frozen at -8 • C for storage.…”

Full-scale testing of low adhesion effects with small amounts of water in the wheel/rail interface

“…Far more work has been carried out to study the effects of leaves than wet-rail. A recent review encapsulates much of this [2]. Investigations have been carried out to characterise the role of water on friction (for example see [3]).…”

Assessing the effectiveness of traction gels using full-scale and field testing

“…Leaves are blown or fall directly onto the railway and stick to the railhead in wet conditions. Chemical reactions between the leaf material and the steel then take place 1,2 and black material is formed which can be pushed or washed down the railhead with subsequent wheel passes and rainfall. Figure 1 shows the blackened leaf material in the contact band after recent rainfall.…”