# The Digital Automatic Coupler (DAC): An Effective Way to Sustainably Increase the Efficiency of Freight Transport in Europe

## 1. Introduction

It is in the interest of Railway Undertakings (RUs) to increase freight train efficiency safely. Considering only the longitudinal train dynamics (LTD), i.e., the motion of the vehicles along the track (including the relative motions), there are several ways to accomplish this task. By increasing the train speed, goods are delivered in less time; since the train braking distance increases with its speed, this method is effective if the “braked weight” increases. Towards this end, wagons with improved braking features are needed, or existing wagons should be revamped; alternatively, the train braking distance should be increased. Another way is increasing the wagon mass: more goods are carried within the same time. Currently, in many European countries, the maximum mass per axle is 22.5 T; increasing this mass requires updates similar to those for the train speed increase and an updating of railway infrastructure to carry the higher wagon mass. A third way is the increase of train hauled mass by increasing the train length: the efficiency increases in a way similar to the previous method. The train length increment requires a railway infrastructure update (e.g., the “sidings” renewal to park the trains temporarily) and a technological improvement of the rolling stock to avoid train derailments or disruptions caused by excessive in-train forces. Among these methods, this paper deals with increasing train length and wagon innovations aiming to increase the permissible longitudinal compressive forces and reduce the in-train forces.

## 2. Digital Automatic Coupler (DAC)

The European DAC Delivery Program (EDDP) from Shift2Rail (S2R) is currently developing the Digital Automatic Coupler (DAC), intending to increase productivity in freight operations in a safe way. DAC is the cornerstone of several innovations mentioned in Europe’s Rail Joint Undertaking (EU-Rail). Among these is the implementation of European rail control-command and signalling (CCS), which is of central importance in running a safe, efficient, interoperable, robust, cost-efficient, and reliable rail service in Europe. DAC is part of the on-board CCS and is expected to significantly improve cross-border operations and multimodal customer services. DAC, among other characteristics, is an enabler for the European Train Control System (ETCS) moving block. It aims to transform the European rail freight system and establish fully digital freight train operations.

As DAC is not yet in production, this paper presents an analysis of the first TrainDy simulation results, comparing the efficiency of the various technical means of optimizing freight transport by rail. The basis for this comparison is given by trains having length, hauled mass, and braking position, allowed by UIC IRS 40421.

## 3. Electro-Pneumatic Brake and Mechanical DAC

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One final remark: even with the current TrainDy modification, the EP brake system discharges the brake pipe between the wagons. However, as of DAC level 3, the brake pipe discharge is applied between the connected wagons; this effect is expected to be minor, although it should be experimentally measured.

## 4. Numerical Results

The following train operations were used for the comparison: (i) emergency braking from full acceleration (TEB)—the train operation that is most challenging for LTD and not likely to occur; (ii) emergency braking (EB). The initial speed for all types of braking is 30 km/h, since the friction material of the braking shoes is cast iron (a conservative assumption), and the LTD is enhanced for braking applications from this speed.

#### 4.1. Reference and Train Statistics

The reference train families for this paper are those explicitly allowed by UIC IRS 40421, i.e., trains with hauled mass in the range (1200–1600) T with Long Locomotive regime (LL) (the first five wagons and the leading TU are in freight position G, the remaining wagons are in passenger position P), and trains with hauled mass in the range (1600–2500) T and (2500–4000) T in G; all with train lengths in (640–740) m but without a DAC or EP brake. The first two train families are likely to have empty wagons, and this does not happen for the third train family because of the higher hauled mass. As the results of the following sections show, the higher minimum hauled mass of the last train family is beneficial in terms of safety.

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Note that the same trains are used and only the technology (or the brake position) is changed in the following sections: therefore, the effect of each change (e.g., radio or DAC) is enhanced. The next section shows the existence of other train families safer than those explicitly allowed to operate (taking as reference the (1200–1600) T in LL), as the aim of the “relative approach” methodology, envisaged by UIC IRS 40421, is to find new safe trainsets.

#### 4.2. Traction Units in Radio Communication

When a third TU is placed at the end of the trainset, there is a beneficial effect on the LCF because the BP is vented from the train end, and this stretches the train reducing the LCF. In this condition, to have a fair comparison with the same train family without the third TU at the end (e.g., 2 × (2501–4000) T), the acceleration phase for TEB operation is accomplished with 2/3 of the maximum power: thus, the train families employ the same power during the acceleration. In fact, the LTD is affected by the initial energy of the train before the EB application. Increasing the total power, having three TUs at their maximum power, the comparison between the train families in terms of in-train forces is also affected by the different initial potential energy (i.e., the couplers are more stretched or compressed). Therefore, the presence of the third TU is not the only difference in the comparison.

The train families 2 × (2500–4000) T show that it can safely increase the hauled mass up to 8000 t (with a train length up to 1500 m) using three TUs and with all vehicles in brake positions G and P, respectively, using GSM-R and LTE radio. Even if this figure shows several possibilities of safe trains with radio communication, they can be many more, e.g., reducing the overall length or coupling trains with different lengths and hauled masses. An example is given by 2 × (1600–2500) T, LL/P, SW, i.e., a train made by coupling two (1600–2500) T trains, the first in Long Locomotive (LL) and the second in passenger position (P) with a remote third TU; this is less safe than the reference UIC IRS 40421 (1200–1600) T in EB operation. A small reduction of the length and/or the mass of the second train makes this train family safer.

#### 4.3. EP Brake and Mechanical DAC (DAC2)

This section demonstrates other possible ways of increasing the hauled mass by employing the EP brake (available also from DAC3 level) and mechanical DAC (DAC2). When a train is formed by coupling two trains and only the mechanical DAC (DAC2) is available, the two TUs must be radio-connected via GSM-R or LTE.

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