The remarkable energy efficiency of freight trains is partly due to the low aerodynamic drag of railcars which draft one behind another. Yet large gaps remain between railcars due to the operational realities of switching, couplers, turning radiuses, Intermodal container stacking and other causes. These gaps increase aerodynamic drag by allowing air to enter and strike the front face of following cars. Previous studies have shown that over 30% of the drag on Intermodal trains is likely due to inter-car gaps.
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​Many attempts have been made to address the drag from these gaps in the past. Previous solutions such as closing, shortening or filling in these gaps have been shown to reduce drag but can be very difficult to integrate into rail operations, and bidirectionality is critical in the rail industry as railcars are made to function going in either direction.​​
At first glance, optimizing freight train aerodynamics might seem counterintuitive. Freight trains are inherently unaerodynamic, and they typically operate at relatively low speeds. However, the sheer number of miles they cover each year - particularly in the U.S. - combined with their lack of aerodynamic refinement, presented a major opportunity. Even small efficiency improvements could translate into substantial fuel savings over time.
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In this article, we will uncover the insights on the performance of the SideRider, a small deflector a few inches in height placed on the sides of enclosed Autoracks can successfully redirect air away from the gap reducing pressure drag on the front face of the following car and total drag at the train level. These deflectors operate without entering the inter-car gap, and their attachment should not create any new operating or maintenance work afterwards making this solution feasible for railroads.
How the SideRider Works
Freight trains have two key aerodynamic characteristics:
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The shape of each railcar is primarily dictated by operational requirements, with little consideration for aerodynamics.
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Large gaps exist between cars for functional reasons, such as couplers, switching, and turning radii.
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These factors contribute to one of the biggest sources of aerodynamic drag in freight trains. High-energy air is drawn into the inter-car gap due to the low pressure between wagons, where it then impacts the front face of the following car. This impact creates a high-pressure zone that significantly reduces the aerodynamic benefits of drafting.
The patent-pending SideRider is a small deflector, only a few inches tall, placed on the sides of the railcar adjacent to the inter-car gap using an adhesive.
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Designed for bidirectional operation, the SideRider redirects high-energy airflow away from the inter-car gap, preventing the formation of high pressure on the front face of the following car. This leads to a significant reduction in drag at the gap.
This deflector has proved to be very effective in reducing the drag at the inter-car gap, as the picture below shows, reducing the drag in the gap by almost 30%.
Trade-offs of the SideRider
Despite its effectiveness, the SideRider introduces some trade-offs:
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The deflector itself generates a small amount of drag.
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By redirecting airflow around the gap, it drastically lowers the pressure inside the gap (see image below).​​​
​​​This pressure drop has two notable side effects:
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The drag on the back face of the leading car increases due to the stronger suction effect.
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Low pressure in the gap pulls airflow from other areas—such as the roof and underfloor—into the gap, increasing drag in these regions.
As a result, while the SideRider achieves significant savings at the inter-car gap, the overall reduction in train drag is somewhat mitigated by these secondary effects.
Wind Tunnel Testing
The numbers were there in CFD, but we decided a wind tunnel test was needed to corroborate our CFD findings, and we came back to TU Berlin wind tunnel, building a modular autorack at 1:20 scale with forces and pressure taps instrumentation to get a better picture of the performance of the deflector and compare our CFD simulations.
The results were highly encouraging, not only did we achieve strong CFD-to-wind tunnel correlation, but we also reinforced our confidence in the CFD methodology, allowing us to proceed with the final development phase of the SideRider. The best performing deflector provided a CdA reduction in car 1 of -12.5% in the wind tunnel while our CFD simulations predicted a reduction of -13.2%. These results validate our CFD simulations which predict a potential CdA reduction of -5% with the latest SideRider design.
Porous SideRider
The solution came with the latest update to the SideRider design: the Porous SideRider.
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By introducing perforations in the deflector, several key improvements were achieved:
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High-energy airflow along the train’s sides is still redirected around the inter-car gap, preventing it from striking the front face of the following car.
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Low-energy airflow near the railcar walls is allowed to pass through the deflector and enter the inter-car gap, increasing the base pressure inside the gap.
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A higher base pressure reduces the drag on the back face of the leading car, while also preventing excessive airflow from other areas—such as the roof and underfloor—from being drawn into the gap.
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The perforations also lower the drag generated by the SideRider itself, especially on the deflectors positioned at the front of each car and facing backward.
The picture on the left, showing the delta in Total Pressure Coefficient respect baseline Autorack, shows how the SideRider is effectively avoiding high energy flow to enter the inter-car gap and collide with the front face of the drafting car.
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The picture on the right, showing the delta in Pressure Coefficient respect baseline Autorack, shows the reduction in pressure in front of the front face of the following car, reducing the drag in the inter-car gap.
With this design refinement, we preserved the aerodynamic benefits of the SideRider while minimizing its drawbacks.
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Current CFD simulations estimate a total drag reduction of 5.9% for autoracks.
Train Performance Simulations
Sixteen train performance simulations were run with varying combinations of uphill/downhill, empty/loaded, straightline/crosswinds and high/low braking losses using the Altrios open source TPS.
Energy savings totaled 1.53% across the 16 runs, which should mean $340 of fuel per equipped car every 80,000 miles traveled at a c-rate of 1 and $3.60 per gallon
The results of the simulations have been used to estimate fuel and emissions savings per thousand miles depending on the amount of autoracks equipped
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Fuel savings of around $1.7m & 4900 tons of C02 emissions every 100.000 miles travelled are estimated.
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There results have been obtained from a case study done for a client with a fleet of 4000 autoracks.
Conclusions
Aerodynamic drag from inter-car gaps is a significant source of inefficiency in freight trains, particularly in Intermodal and Autorack configurations. Traditional solutions, such as closing or minimizing these gaps, have proven impractical due to operational constraints.
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The SideRider was developed as a simple and effective solution to address this issue, reducing drag at the inter-car gap without interfering with rail operations. However, initial designs introduced secondary effects, such as increased drag on the back face of the leading car and additional flow disturbances in other areas.
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The Porous SideRider successfully mitigates these drawbacks by allowing low-energy airflow to enter the gap while still redirecting high-energy air away. This innovation maintains the benefits of drag reduction while improving overall train aerodynamics.
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With ongoing optimization and further testing, the SideRider concept has the potential to significantly enhance the energy efficiency of freight trains, contributing to reduced fuel consumption and lower emissions in the rail industry.
