Sustainability is a topic discussed worldwide focusing on actions to minimize climate change and global warming. Given this, the United Nations has proposed 17 Sustainable Development Goals (SDGs), which involve the implementation of strategies to combat climate change and its impacts, among other objectives. Adding to this, the European climate law stated that the European Union (EU)countries must reduce their greenhouse gas emissions by at least 55% by 2030 [1].

Road transport is the mode that most suffers from congestion, and freight transport contributes to the worst situation [2]. Due to this, the search for more sustainable long-haul freight transport is a factor that positively influences the goals cited above, mainly seeking strategies to reduce the share of roadway transport in the modal distribution, improving others such as inland navigation. Given this, governments and policymakers have made efforts to shift freight traffic from road to rail and waterways, particularly in Europe, to support greener modes of transport [3].

A transportation chain is divided into three segments: pre-haul (or first mile), long-haul, and end-haul (or last mile) [4]. In most cases, pre-haul transport and end-haul transport are included in roadway transport (possibly combining different vehicles). In contrast, plenty of different modes can be considered for long-distance transport such as road, rail, air, and water. Moreover, non-road modes of transport can potentially increase their share of freight transport based on internal and external influences (cost, service reliability, capacity, availability) [5]. Also, according to the authors, the water freight transport sector can grow significantly based on the enhancement of the supply and its integration with other modes. These improvements can have a direct impact on logistic chains, such as cost reductions and better efficiency.

Following this, the search for solutions to make freight transport more sustainable has been going on for decades, presenting concepts such as multimodality, intermodality, and—the most recent one—synchromodality. The multimodal concept is less restrictive [6] since it only requires the inclusion of two or more modes of transport. In this line, the concept of intermodality adds the requirement that the cargo remains in the same cargo unit throughout the journey without the need for extensive handling or repackaging [7]. Even over the years, these solutions do not represent the majority of the total freight share [8]. A model to compare intermodal and synchromodal transport impacts was created by [9]. A study case on this model found that synchromodal systems tend to improve the transport service level, capacity utilization, and modal shift but do not specifically reduce delivery costs (lower overall shipping costs and higher handling costs). Due to this, synchromodality is a possibility to improve the performance of freight [8] in the current era.

Synchromodality, or synchromodal transport, represents an innovative approach searching to optimize the movement of goods by considering the most efficient and cost-effective transport modes based on real-time conditions. This approach enables opportunities to integrate various transportation modes, such as road, rail, air, and sea, into a synchronized network. The main pillar of synchromodal transport is to minimize cost, time, and environmental impact while integrating the vehicles’ capacity, transportation modes, and customer demands [10]. Synchromodality involves the decisions of different stakeholders such as shippers, freight forwarders, and carriers [11], but these decisions are not predefined in advance and are decided using real-time information from the infrastructure and transport providers ^[7][12][7,12].

Synchromodal transport is an important factor in freight movement and logistics, as it presents a more sustainable type of transport ^{[9][13][14][15]}[9,13,14,15], improves benefits from a societal and environmental perspective [9], and allows a smarter utilization of available infrastructure and service resources ^[12][13][16][12,13,16]. Additionally, even though the origin of synchromodal transportation is related to environmental issues, 70% of the quantitative research focuses only on economic goals [17]. The evolution of the synchromodality concept is shown in Table 1.

From Table 1, it is clear that the concept of synchromodality has evolved. Initially, it was described as an efficient and synchronized combination of transportation modes. However, as time passed, the definition expanded to include factors like real-time information, integrated planning, cooperation among stakeholders, and the use of information and communication technologies (ICT) and intelligent transportation system (ITS) technologies. The definitions suggest that the concept is seen as a way to optimize freight transportation while also considering environmental and economic sustainability and reflecting the dependence of synchromodality on the coordination and cooperation of stakeholders in the supply chain, data availability and integration, and technology to improve decision making.

The implementation of synchromodality in real life has found numerous challenges ^[8][12][15][8,12,15]. The global impact of multimodal transport on the supply chain is one of the reasons why the modal split is so difficult to implement [3]. Indeed, synchromodal systems might imply results regarding the transport system performance in terms of economic, societal, and environmental impacts [9].

Moreover, synchromodality can be completely influenced by internal and external issues [5]. The internal issues are related to cost, service reliability and punctuality, capacity available, and industry skill, among others, while the external influences are connected with policies and funding, environmental topics, modification of the supply chain structure, average distances for freight movement, land use planning, etc.

The synchromodal transport challenges were discussed by [15] and were then divided into five different layers: logistics layer, transaction enablers, governance enablers, institutional enablers, and cultural enablers.

Regarding the institutions, transportation companies, logistic providers, port authorities and terminals, technology providers, and intermodal terminals are examples of companies that are part of the freight synchromodal decision process. In summary, the challenges in the implementation of synchromodal transport include various and distinct aspects. From a logistics point of view, more than aligning the demand needs, the integration among the transport service levels is quite important. From the transaction, governance, and institutional enablers, considerable changes are needed, and these changes need to be aligned with the cultural enabler.

In addition to the factors mentioned above, synchromodality is also about understanding the stakeholders’ preferences. A complete understanding of preferences contributes to reducing costs and improving the service level through the provision of customized services [11]. According to the authors, the most effective transport plan requires complete understanding of the heterogeneity preferences of service from shippers.

A hierarchy of decision problems is discussed in the research performed by [8] that aims to list the challenges found in the implementation of synchromodal freight transport systems.

Strategic, tactical, and operational planning problems in synchromodality implementation were also taken into account by [4]. Regarding strategic planning problems, the authors cited problems such as mode definition, multi-commodity, allocation, direct shipment, capacity, transshipment, and schedule issues, among others. Also, there are issues related to fixed schedules, assets, empty flows, elastic demand, split delivery, additional scheduling issues, synchronization, and decentralized decision making in tactical and operational planning problems.

Among the three most common means of freight transport (roadway, railway, and waterway), rail or water transport is generally cheaper and more environmentally friendly, but its services are not as frequent and fast as road travel, causing a lack of flexibility in delivery quantity, frequency, and schedule [3]. As a result of this, logistics operators tend to consider that these modes can negatively impact the supply chain. According to [15], the modal choice among the three options will be based on the relative importance of transport time and costs.

The main risk of synchromodal transportation is the limited, or fixed, schedules of the transport modes because this decreases its flexibility [10]. In this line, a good synchronization of services provided by the different modes of transport, adding complementary information about the inland terminals’ services (transshipment and storage information) is one way to reduce waiting times and overall transportation costs (eliminating intermediate storage, for example) [15].

The concept of synchromodality, which proposes the efficiency and flexibility of transport, is built considering cooperation among all stakeholders along the transport chain [12]. So far, there are numerous challenges in implementing synchromodality because it requires collaboration and coordination between different stakeholders and a constant sharing and monitoring of information.

The manner of cooperation between the parts involved, as well as the acceptance of a central network organizer, is one of the many inconsistencies existing in the area of synchromodal transportation [7]. Other researchers talk about horizontal cooperation, real-time changes, and necessary mind-shift as barriers to synchromodal transportation [15].

Moreover, it is important to set up standards for data sharing between stakeholders, ensuring the high quality and veracity of information [12], so this stakeholder collaboration could promote on-time delivery and the integration of different levels of planning tending to provide more reliability, flexibility, and sustainability ^[4][12][4,12].

Regarding the cultural issue, [17] states that synchromodality is one of the main topics in discussions about sustainable transport logistics; however, awareness among stakeholders needs to be strengthened. Additionally, beyond collaboration, active participation in the process of a mental shift is required by the involved parties to ensure the necessary freedom for fulfilling a-modal transport services [12].

Based on the three dimensions of synchromodality defined by [8], the triggers of synchronization in each one were defined by [19] as follows:

• Moving resources: Service schedule, vehicle capacity and availability, operating hours.
• Customer demands: Size, pick-up and delivery time windows, delivery due time.
• Stationary resources: Link availability, terminal capacity, terminal operating hours.

A technical challenge of synchronization involves the implementation of integrated information solutions to minimize container wait times at origin terminals and waiting times for transport services at destination points [8]. In unplanned cases, the transshipment between modes or loading and unloading the cargo could take hours of waiting. A modal considering constraints was created by [8] in favor of determining an optimal schedule of multiple transport modalities for a specific time. It considered constraints regarding capacity, flow, delivery time, service sequence, and infrastructure usage. Consequently, it offers a comprehensive depiction of the constraints associated with the operational synchronization of both moving and stationary resources. An approach that considers estimations of the demand and focuses on planning truck flow and barge schedules is also considered [20].

From the perspective of a shipper who wants to transport a specific freight from an origin to a destination, minimizing transport costs, the “a-modal” reservation approach enables the transportation provider to determine the transportation modes and vehicles to be employed in a specific transportation request, so the shipper buys the transport service (no a-slot in a specific mode) ^[21][22][21,22]. Additionally, it allows the service provider to make any adjustments during the route. From a synchromodal transport point of view, the time of decisions realized at the tactical level becomes closer to the time of decisions at the operational level [6] when the route and mode definition can be replanned based on online information [21]. In this case, solutions that consider a prediction of traffic levels, constraints existent, and transport options available have been on focus to a more practical synchromodality approach ^{[8][20][21][22]}[8,20,21,22] by using models that provide different scenarios of mode shift based on cost and time calculations, among other factors.

In summary, the literature review reinforces that synchromodality offers significant promised for enhancing the sustainability and efficiency of freight transportation, but there are substantial issues in its implementation. The main conclusions about the concept, impacts, and challenges of this section can be summarized as follows:

(i)

The synchromodality concept supports mode integration and dynamic mode switching based on current transport conditions.

(ii)

Synchromodality has the potential to reduce costs, reduce times, and support a modal shift away from road transport, consequently alleviating congestion and reducing emissions.

(iii)

Besides other challenges, the implementation of synchromodality deals with real-time information and decision making.