Sand delivered by a railway sander is not simply “dry sand.” Its function is to help restore adhesion when water or other contaminants degrade the wheel–rail interface. For a buyer, qualification therefore becomes a question of particle-size distribution, consistency, flowability and compatibility with the sanding system, rather than a search for the highest isolated SiO₂ value. Tribology research also shows a trade-off: particle characteristics that affect adhesion can also affect wear and surface damage [2–4].
In Morocco, ONCF provides the context of a railway network operated at national scale [1]. That does not reveal the specification of its sanding equipment, its suppliers or operating parameters. This article therefore treats the subject as a raw-material qualification method to be checked against the actual rolling-stock specification.
1. Why does sand improve wheel–rail adhesion?
Available adhesion directly limits the traction and braking force that can be transmitted between wheel and rail. When the interface is contaminated, hard sand particles can act as a third body that disrupts or removes contamination and changes contact conditions [2,6].
British railway investigations illustrate the operational importance of the mechanism: RAIB describes train-borne sanding as an established mitigation for low adhesion and explains that sand is delivered towards the wheel–rail interface to help restore traction or braking performance [6]. This explains the function of sanding; it is not a Moroccan raw-material specification.
Key point: the useful criterion is not “silica sand = good adhesion.” A candidate material must be assessed through how it is stored, delivered, fragmented and behaves in the wheel–rail contact of the actual system.
2. Particle size matters, but not independently of the system
A twin-disc study compared four particle-size distributions and several slip levels. Under that protocol, larger particles could produce higher adhesion, but also more wear depending on slip [2]. The result is useful for understanding the trade-off, not for imposing a universal size.
A later study compared so-called standard sand at 0.6–2.0 mm with fragments at 0.1–30 µm in its own test protocol. The authors observed that fragments could maintain adhesion enhancement while reducing some damage, although rolling-contact-fatigue damage occurred under certain water–fragment conditions [3]. These values are experimental conditions, not a Minerallum purchasing range or a Moroccan standard.
Work published in 2023 also concluded that particle size, shape and hardness affect tribological performance [4]. Procurement should therefore retain several dimensions rather than reducing qualification to one sieve.
| Material parameter | Why measure it | What not to conclude |
|---|---|---|
| Particle-size distribution | Delivery compatibility and contact behaviour | That one published range is universal |
| Fines | Lot drift, dust and potential flow effects | That one limit fits every vehicle |
| Shape / angularity | Mechanical interaction and fragmentation | That one shape is always optimal |
| Hardness / crushing behaviour | Influence on fragmentation and contact | That “harder” is always better |
| Moisture | Storage and flowability | That a generic limit replaces the system specification |
3. Drying and flowability: qualify deliverability, not chemistry alone
Sand only works if it reaches the interface when demanded. Supplier qualification should therefore treat it as a granular material that must be stored and then conveyed through a delivery device.
Moisture is a relevant control parameter because it can change granular behaviour and promote agglomeration. However, the reviewed sources do not justify a universal moisture percentage for all railway sanders. The correct limit is the one validated by the relevant equipment specification and testing.
The same caution applies to fines. An increase in fines can indicate production, handling or fragmentation drift. It should be monitored using a repeatable method without arbitrarily converting a laboratory result into a universal acceptance criterion.
4. More sand or larger grains do not automatically mean “better”
Sanding behaviour depends on material + feed rate + speed + contamination + slip + delivery geometry. A 2025 twin-disc study found that deposited particle density could strongly influence adhesion restoration and damage, and that the effect of particle size depended on that density [7]. This reinforces why single-parameter optimisation is weak.
The opposite risk also exists. Research has investigated solid lubrication and electrical insulation caused by sand under some dry-contact conditions. Railway systems therefore need to manage adhesion, wear, electrical detection and delivery together [2–4].
For procurement, the consequence is straightforward: a material datasheet alone cannot promise braking or traction performance.
5. How should a railway sanding-sand lot be qualified?
A robust purchasing procedure can be organised into six steps.
- Obtain the rolling-stock or sander specification. Identify test methods, particle-size distribution, fines limits, delivery condition and storage requirements.
- Define a sampling method. The sample should represent the lot rather than only the surface of a bag or stockpile.
- Measure particle size consistently. Retain the full distribution instead of only a nominal size.
- Control moisture and physical condition. Look for drift that could affect storage and delivery according to contractual limits.
- Track only complementary properties that are actually required. Shape, hardness, chemistry or mineralogy matter contractually when linked to the system or control plan.
- Validate in the railway context. Published tribology tests provide mechanisms and trends; final acceptance remains linked to applicable equipment and procedures.
This is more defensible than a “premium railway sand” claim based on purity alone.
6. What documents should a supplier provide?
To reduce ambiguity between a commercial offer and a qualified product, the supplier dossier can include:
- lot identification and traceability;
- particle-size result using the contractual method;
- moisture or delivery condition when specified;
- fines control where relevant;
- sampling method;
- test frequency;
- re-test criteria after storage or transport where needed;
- safety data sheet where the regulatory framework requires one.
Chemical composition may be part of the dossier, but it does not replace particle-size control or functional validation.
7. Morocco: separate railway context from material specification
ONCF reports 2,295 km of lines and 3,350 km of track in 2024 [1]. These figures establish only the scale of the national railway context. They do not prove the use of a particular silica-sand grade, particle-size distribution or purchasing volume.
That distinction matters: a responsible technical article can explain how to qualify railway sanding sand in Morocco without inventing an operator’s practices, equipment settings or suppliers.
8. Procurement decision: focus on compatibility and consistency
Railway sanding sand is a functional component of an adhesion-management system. Particle size matters, but its effect depends on feed rate, slip, contamination and contact conditions. Particles can improve adhesion while also influencing wear, damage and, under some conditions, electrical insulation [2–6].
The sound purchasing logic is therefore to start from the system specification, then qualify particle-size distribution, delivery condition, lot consistency and any complementary properties that are genuinely required. Published test values help explain mechanisms; they should not become universal thresholds without validation.
References
[1] ONCF. ONCF - network and key figures.
[2] Arias-Cuevas et al. A laboratory investigation on the influence of the particle size and slip during sanding on the adhesion and wear in the wheel–rail contact. Wear, 2011.
[3] Shi et al. Laboratory investigation on the particle-size effects in railway sanding: Comparisons between standard sand and its micro fragments. Tribology International, 2020.
[4] Skipper et al. Quantifying the effect of particle characteristics on wheel/rail adhesion & damage through high pressure torsion testing. Tribology International, 2023.
[5] Maramizonouz et al. CFD–DEM modelling of particle entrainment in wheel–rail interface: a parametric study on particle characteristics. Railway Engineering Science, 2025.
[6] Rail Accident Investigation Branch. Collision between two passenger trains near Talerddig, 2026.
[7] Zhang et al. Optimization of sanding application parameters based on the wheel-rail adhesion restoration effect and surface damage. Wear, 2025.