Overview
As the rail industry moves toward Zero Emission Vehicle (ZEV) operations, additional developments have arisen – dual mode and diesel hybrid locomotives, that potentially can provide a bridge between the current technological phase and full Zero Emission operations. The current phase is the conversion of the locomotive fleet to Tier IV locomotives. California Regional Rail Operators and Amtrak either have a 100% Tier IV fleet or are moving quickly to full conversion. The freight railroads are at a slower pace and are just now starting to order larger numbers of Tier IV locomotives.

Before outlining the new alternatives, an overview of the current phase. Diesel fueled Tier IV locomotives bring an 85% reduction in criteria pollution and particulates but yield only a modest reduction in Green House Gas (GHG) emissions. However, California Regional Rail and California Corridor operators have gone a step further, adopting recycled vegetable oil and animal fats as fuel. This enhancement yields additional reductions in criteria pollution and particulates and most importantly, an 80% reduction in GHG emissions. This reduction in GHG emissions is a result of the double use of fuel feedstocks.

To weigh the importance of the reduction in emissions, one must consider the impact of the pollutants. Criteria pollution is ground level pollution, primarily Nitrogen Dioxide (NO2) and Particulates (PM2.5 and PM10). Nitrogen Dioxide reacts with Ozone (O3) to form smog. PM2.5 are soot particles so small that they can penetrate deep in the lungs and enter the bloodstream and can cause serious health risks. PM10 are larger soot particles that lodge in the upper respiratory system (like pollen) causing upper respiratory and/or cardiovascular issues. Greenhouse gas emissions (GHG) rise high in the atmosphere trapping heat and raising global temperatures resulting in more extreme and frequent severe weather events.

Because of the importance of continuing to reduce harmful emissions, it is critical to keep moving forward toward Zero Emission operations. New dual mode and hybrid diesel locomotives can provide a bridge between Tier IV diesel locomotives and Zero Emission operations. This bridge phase is especially important given the technical complexity and fuel challenges resulting in the current choice for ZEV, hydrogen, stalling.

Emerging Technology: Dual-Mode and Hybrid Locomotives
Two classes of dual-mode locomotives are being constructed for Amtrak by Siemens at its Sacramento plant. In both cases the power modes operate separately based on the choice of the engineer.

Diesel Battery
The first is a diesel battery power consist that Amtrak will use on Adirondack, Empire Service, Maple Leaf and Ethan Allen trains. All of these routes use the Empire Connection tunnel to access Penn station and Sunnyside Yard where the trainsets are serviced overnight. This requires trains on these routes to avoid using diesel power in dense New York City neighborhoods and through the underground tunnels of the Penn Station/Sunnyside Yard complex.

Older Empire Service locomotives used a third rail to access Penn Station. The new Siemens diesel battery locomotives take advantage of advances in battery technology in an initiative to evaluate the viability of a diesel battery option. The diesel is Siemens newest variant of the Charger, the ALC-42E which has an electric bus to allow current from the battery to feed the locomotive traction motors and for the diesel engine to charge the batteries. The battery car contains the battery packs and Business Class seating. Siemens’ design concept involves placing the heavy batteries in a separate car which means less alteration to the basic Charger locomotive design, spreads battery weight over additional trucks for better wheel/rail dynamics and reduced track stress. It also provides the flexibility to extend ZEV range by adding more battery packs. While this highly targeted (Penn Station tunnels) prototype will have limited pollution reductions, those reductions are in sensitive locations, and the range of this concept could be extended with additional battery packs. For longer range concepts, regenerative braking can be used to recharge the battery. Please note that while the illustration in Figure One is labeled “hybrid” the Airo trainset is in fact dual-mode, with each power system separate and operating independently based on the engineer’s choice.

Diesel Overhead Catenary
Amtrak Airo Diesel + APV
Amtrak has another type of dual-mode power consist under construction by Siemens, it is a diesel power consist that can be powered by overhead catenary. The result is a train that can operate seamlessly between electrified and non-electrified routes. This dual mode option pairs the ALC-42E Charger with an Auxiliary Power Vehicle (APV). The APV is a car with Business Class seating and equipped with two pantographs, a main transformer and two powered trucks. The APV will pull power from the overhead catenary on electrified lines transmitting power to its two powered trucks as well as the two trucks on the locomotive via the DC bus.

As with the diesel battery power consist, having the heavy transformer on the APV instead of in the locomotive means a minimally altered Charger design can be used for multiple power options. Also, spreading the weight of the transformer over the two car trucks also results in better wheel/rail dynamics and reduced track stress. Like the diesel battery locomotive, the two power systems are separate and operate independently based on the engineer’s choice.

The Airo trainsets with the diesel Overhead Catenary APV will be used on Vermonter, Keystone Service, Pennsylvanian, Carolinian, Palmetto and Northeast Corridor Regional trains serving Virginia. Having the flexibility of a dual-mode means there will be time saving from avoiding an engine change and loss of train hotel power during the change. Avoiding the track time associated with an engine change will result in more station throughput, especially at Washington Union Station. The major environmental benefit is that the diesel + APV can leverage the NEC electrification and avoids operating a diesel through dense NEC urban areas. Regenerative braking will be used to feed power into the overhead catenary to power other trains. It also prepares the fleet for expansion of overhead catenary i.e. New Haven – Springfield or Washington, DC – Richmond, VA. And there is a place for this type of locomotive in California, operating San Luis Obispo to San Francisco utilizing overhead catenary from Gilroy into the underground San Francisco tunnel and the downtown SalesForce Transit Center.

New Jersey Transit ALP-45DP
The New Jersey Transit Bombardier (now Alstom) ALP-45DP is a dual-mode diesel overhead catenary locomotive. Both the diesel engines, the electrical transformer and pantographs are on the single locomotive. New Jersey Transit operates seventy-two of these dual-mode locomotives representing a significant percentage of its non-electrified operations. These locomotives operate on NJT’s extensive non-NEC branch lines linking into the NEC and allowing a one-seat ride from scores New Jersey communities to Penn Station in New York City.

Needless to say, combining diesel traction and a high voltage electric into one locomotive while meeting Northeast corridor clearance requirements was quite the design effort. Bombardier’s solution was to “wrap” the diesel traction around the transformer using two high-rpm diesel engines on either side of the transformer which is in the middle of the locomotive. The locomotive is capable of changing modes while in motion, although NJT only does this at station stops. Like the Amtrak dual-mode diesel + APV, the major environmental benefit is that the ALP-45DP can leverage NEC electrification avoiding operating a diesel through dense NEC urban areas. Also, by allowing a one-seat ride, NJT has a more attractive product shifting riders from highway mode to the rail mode. Regenerative braking is used to feed power into the overhead catenary to power other trains.

Diesel Battery Hybrid
Via Hybrid Diesel Battery Locomotive
In a striking future vision initiative, Via Rail is bringing innovative technology to the long-distance locomotive market. It has purchased forty-five Diesel Battery Hybrid locomotives from Stadler. Stadler, better known for its EMU and unique rack rail cars, also offers a wide range of electric, diesel and dual-mode locomotives. What is most unique about Via’s Stadler Diesel Battery locomotives is that they are true hybrids with an integrated battery and diesel power package,

Also of note is that these innovative technology hybrids were not purchased to operate along a “close-to-home” corridor route, but along a long-distance route often far from service facilities and subject to very harsh winter operating conditions of bitter cold and fine snow.

With hybrid architecture the diesel engine, battery, traction, hotel (car interior) power, and auxiliary systems are all integrated into a unified system with power generated, supplied, and used in a coordinated balanced system. With the combined power sources of the engine and battery, acceleration, cruising and high-performance on steep gradients is managed to yield a high-level of efficiency. Fuel savings reduction is achieved by derating the diesel engine to optimize its operation at its most efficient RPM level with the battery providing the extra power needed for acceleration and steep gradients. Operating the diesel engine within its most optimal RPM range also extends its lifespan while reducing maintenance.

This fuel saving translates into lower criteria and GHG emissions. Reducing emissions further is regenerative braking, where the traction motors transform braking energy into electricity to recharge the batteries while slowing the train.

At stations and other environmentally sensitive areas the engineer can operate solely in the battery mode, significantly reducing noise and eliminating exhaust emissions. The battery can also enhance operational resilience by providing a “limp home’ ability so the train can reach the next station, road, or inhabited area in case of a diesel engine failure. Overall, Via’s initiative and Stadler’s innovation set a new standard for operational efficiency – lower fuel consumption, cost savings, and reduced emissions.

Hybrid Tri-Mode Trains
Hitachi Great Western Railway Class 802 Trains
There are also initiatives being undertaken outside the US to develop more energy efficient, less polluting trains.

In the United Kingdom, Hitachi, and equipment owner Eversholt have rebuilt existing dual-mode diesel overhead catenary trains into hybrid tri-mode trains

This was accomplished by removing one of the two diesel engines and replacing it with batteries. And this train is a true hybrid with the battery and diesel engine integrated to work in tandem to improve efficiency. Thirty-six trainsets were rebuilt into Tri-Mod Hybrids. The battery is used to supplement the diesel when peak power is required, and it is also used in urban areas and traveling out of stations.

These Tri-Mode trains operate on the Great Western Railway between London and Penzance (300 miles) in the west of England. This route is partially electrified and was operated exclusively on diesel power outside of electrified territory. With the introduction of the Tri-Mode trains, the efficiency is increased over high-power demand route segments and acceleration out of stations the as diesel engine is supplemented by a power boost from the battery. This is estimated to reduce fuel consumption and emissions by approximately 20%.

In 2025 Arriva Group, operator of the Grand Central Railway which connects London and Yorkshire and the Northeast, ordered nine Tri-Mode trainsets (Class 820) from Hitachi for delivery in 2028.