This paper presents a comprehensive systematic review of low-carbon solid waste materials applied in railway sub-ballast layers, aiming to critically assess their mechanical performance, durability, environmental benefits and regulatory readiness. The study addresses the growing need to decarbonize rail infrastructure while reducing dependence on natural aggregates, positioning sub-ballast as a strategic layer for circular economy implementation in ballasted track systems.

A PRISMA-based systematic review methodology was adopted to identify, screen and analyses peer-reviewed studies published between 2000 and 2025. The final database comprises experimental, numerical and field investigations covering mining residues, steel slags, construction and demolition waste, rubberized composites, alkali-activated materials and other industrial by-products applied to railway sub-ballast. Mechanical behavior under cyclic loading, resilient modulus, permanent deformation, hydraulic performance, durability and environmental indicators were extracted and synthesized. In parallel, an international regulatory analysis was conducted to compare sub-ballast specifications across Europe, North America, Asia-Pacific and Brazil, enabling identification of performance-regulation gaps and barriers to implementation.

The review demonstrates that several low-carbon waste-derived materials exhibit mechanical performance comparable to or exceeding that of conventional granular sub-ballast, particularly in terms of stiffness retention, resistance to permanent deformation and degradation under repeated loading. Steel slags, recycled concrete aggregates, slate waste and rubber-modified blends consistently show favorable resilient behavior and enhanced damping capacity, while certain mining residues and alkali-activated granular systems present promising strength and durability characteristics. Life-cycle evidence indicates substantial reductions in embodied carbon and natural aggregate consumption when these materials are adopted. However, current railway standards remain largely prescriptive and index-based, rarely incorporating cyclic performance criteria or carbon metrics, creating a structural disconnect between scientific evidence and regulatory acceptance. This gap significantly limits large-scale implementation despite growing technical maturity.

This study provides the first integrated synthesis focused exclusively on low-carbon solid waste materials for railway sub-ballast, combining mechanical performance, environmental assessment and international regulatory comparison within a unified analytical framework. By explicitly linking laboratory evidence to policy and standardization challenges, the paper advances performance-based pathways for sustainable railway substructure design. The findings offer actionable guidance for infrastructure managers, regulators and researchers seeking to accelerate the transition toward circular, low-carbon rail systems through sub-ballast innovation.

This paper provides a comprehensive analysis of the Brazilian freight railway system, examining the efficacy of the current concession renewal model in light of persistent structural problems such as market concentration, cargo dependence on export commodities and underutilization of the network. Situating Brazil within the broader international debate on railway reforms, the paper evaluates whether the ongoing early renewal of concessions can deliver a more diversified and competitive freight system.

The study adopts a sequential mixed-methods research design that integrates longitudinal quantitative analysis with qualitative institutional and policy evaluation. The quantitative component examines time-series indicators published by ANTT, DNIT and INFRA S.A. from 1999 to 2023 to identify structural patterns in traffic growth, investment, safety and market concentration. The qualitative component employs a process-tracing logic to reconstruct the evolution of concession renewals and the implementation of Railway Law 14.273/2021, drawing on concepts from regulatory economics, institutional theory and industrial organization. These empirical streams are synthesized through an analytical framework that connects three dimensions - regulatory design, market structure and system performance - allowing for a systematic assessment of how Brazil’s institutional configuration shapes incentives, competitive dynamics and network utilization.

The analysis confirms that the early renewal of concessions has successfully secured substantial private investment for capacity expansion on existing trunk lines. However, it has perpetuated the vertically integrated model, reinforcing the market power of incumbent operators and failing to significantly promote intramodal competition or cargo diversification. The system remains dominated by iron ore and agricultural commodities, with general cargo representing a minuscule share. The new authorization regime and short-line railway policies present a viable pathway for market opening but face significant operational and institutional barriers to implementation.

This research offers a timely and critical assessment of a pivotal moment in Brazilian railway policy. It moves beyond a simplistic evaluation of volume growth to a structural analysis of market failures and the interplay between concession renewal and regulatory innovation. The findings provide actionable insights for policymakers in Brazil and other emerging economies seeking to balance private investment with public interest goals in railway infrastructure, highlighting the necessity of complementary, pro-competitive measures alongside financial investment.