A battery electric multiple unit (BEMU), battery electric railcar or accumulator railcar is an electrically driven multiple unit or railcar whose energy is derived from rechargeable batteries that drive its traction motors.
Prime advantages of these vehicles is that they do not need to use fossil fuels such as coal or diesel fuel, emit no exhaust gases and do not require the railway to have expensive infrastructure like electric ground rails or overhead catenary. On the down side is the weight of the batteries, which raises the vehicle weight, and their range before recharging of between 300 and 600 kilometres (186 and 373 mi). Currently, battery electric units have a higher purchase price and running cost than petrol or diesel railcars, needing one or more charging stations along the routes they operate, unless they operate on a mixture of electrified and unelectrified track being charged from the electrified track.
Battery technology has greatly improved over the past 20 years broadening the scope of use of battery trains, moving away from limited niche applications. Despite higher purchase and running costs, on certain railway lines battery trains are economically viable as the very high cost and maintenance of full line electrification is eliminated. From March 2014 passenger battery trains have been in operation in Japan on a number of lines. Austria has overhead wire/battery trains which became operational in 2019. Britain successfully trialled fare paying passenger hybrid overhead wire/lithium battery trains in January and February 2015.
Experiments with accumulator railcars, as they were originally called, were conducted from around 1890 in Belgium, France, Germany and Italy. In the USA, railcars of the Edison-Beach type, with nickel-iron batteries were used from 1911. In New Zealand, a battery-electric Edison railcar operated from 1926 to 1934. The Drumm nickel-zinc battery was used on four 2-car sets between 1932 and 1946 on the Harcourt Street Line in Ireland and British Railways used lead-acid batteries in a railcar in 1958. Between 1955 and 1995 Deutsche Bahn railways successfully operated 232 DB Class ETA 150 railcars utilising lead-acid batteries.
A number of tramway manufacturers are offering battery tramcars that combine the traction battery with a supercapacitor that will be charged at each stop. The main motivation for the usage of battery-powered tramways is to avoid overhead wires across a city. Using boost charging at each stop allows to lower the size of the required traction battery. This technology is hoped to be transferred to full trains.
The CAF Urbos 3 tramways was ordered for the Seville MetroCentro with the Acumulador de Carga Rápida system which uses short overhead wires at each stop for charging. The Seville trams have been operating since Easter 2011.
Siemens three car trams are to be used for Education City in Doha, the capital of Qatar. The opening was scheduled for autumn 2015 but has been delayed until 2019. No overhead wires will be installed, as the 10 Avenio trams will be powered by the Siemens Sitras HES system (Hybrid Energy Storage), a combination of a supercapacitor and a traction battery that will be charged at each stop through an overhead conductor rail.
A Combino tramway using four car tramsets equipped with the Sitras HES system has been in regular service from Almada to Seixal, Portugal, since November 2008. It is capable of running up to distances of 2,500 metres without overhead wires.
The Byron Bay Train service in Byron Bay, New South Wales operates a heritage 600 class railcar which was converted to solar power for use on a formerly disused section of line through Byron Bay. It is believed to be the world's first solar-powered train.
The Austrian Federal Railways has purchased 189 Siemens Desiro ML trains from 2013 to 2020. One of these trains was converted to a battery electric multiple unit and branded as cityjet eco. The battery/electric version of the Siemens train is equipped to operate with batteries and overhead wires, with a battery only range of 80km reaching a maximum speed of 100 km/hr in battery mode. The trains are to be tested on regional and suburban rail lines on electrified and unelectrified track. The test services began in September 2019 on the Kamp Valley line between Horn and St. Pölten.
In 1887 the first German accumulator railcars were placed in service by the Royal Bavarian State Railways. Their development continued with the pre-Second World War classes ETA 177 to 180, the post-war DB Class ETA 176 (later 517) and finally ended with DB Class ETA 150 (later 515). The latter were used until 1995 having been since modernised into the Nokia ETA, painted light grey and green and deployed onto the so-called Nokia Railway (timetable number RB 46), nowadays the Glückauf-Bahn from Gelsenkirchen via Wanne-Eickel to Bochum.
Between 1932 and 1949, two battery electric trains ran successfully between Dublin and Bray. Two additional trains were built and placed into service in 1939, also running until 1949. The train's batteries were charged at each terminus via an overhead pickup. The train could reach 60 mph/105km/h, though service speeds were typically limited to 40 mph/66 km/h. 
In 2019, Iarnród Éireann announced they would purchase 250, with an option of up to 600, new carriages over a 10-year period, for use on the DART network. The order will consist of both electric multiple units and battery electric multiple units.
NE Train Smart Denchi-kun
In Japan, JR East tested a "NE Train Smart Denchi-kun" battery electric railcar from 2009. This vehicle is capable of operating under 1,500 V DC overhead wires or on battery power alone for a distance of up to 50 km away from an overhead power supply. The batteries are charged via the pantograph either when running under an overhead electric supply or at a specially built recharging facility.
From March 2014, a two-car battery electric multiple unit, the EV-E301 series with 1,500 V DC overhead wire and battery power capability, entered revenue-earning service on the 20 km (12 mi) long non-electrified Karasuyama Line.
Since March 4, 2017, battery electric trains have completely replaced diesel multiple units on this route. There are four two-car sets in operation, each set equipped with a 190 kWh lithium-ion battery.
The trains recharge their batteries at Karasuyama Station (the end of the line) via their pantographs, using a rigid conductor bar placed where the overhead wires would be, connected to the local electric grid. At the other end, at Hōshakuji, the Karasuyama Line meets the electrified Tohoku Main Line. The trains then continue over the electrified Tohoku Main Line to Utsunomiya, where passengers can change to Shinkansen high-speed trains.
From April 2016, JR Kyushu started trial operation of a two-car BEC819 series BEMU train, nicknamed "DENCHA", on part of the Chikuhō Main Line, with revenue-earning service scheduled from October of the same year, and six more trains planned to be introduced on the line in spring 2017. It can also be powered via overhead AC power line. The BEC819 series battery/electric trains became operational on the through services on the Fukuhoku Yutaka Line in October 2016, which is electrified at 20 kV AC and the non-electrified Chikuhō Main Line, known as the "Wakamatsu Line", between Orio and Wakamatsu.
The battery capacity is 360 kWh.
In 2019, a total of 11 new BEC819 trainsets were introduced on the Kashii Line, replacing all diesel multiple units previously operated on that line. This brought the total number of BEC819 trainsets in revenue service to 18.
A new EV-E801 series two-car BEMU train was introduced on the 26.6 km (16.5 mi) long non-electrified Oga Line in Akita Prefecture in March 2017. It shares some characteristics with the BEC819 trains: it uses a 360 kWh battery and is recharged from a 20 kV AC overhead supply instead of a 1,500 V DC overhead supply used by the earlier EV-E301 trains.
Auckland Council planned to purchase 17 battery/electric trains from CAF in Spain, to operate between Britomart and Pukekohe. From Britomart the trains would operate on electric overhead wires to Papakura. From Papakura the trains would switch to battery mode for the journey to Pukekohe. Operation was to start in 2019.
Battery electric railcars used by British Rail included the British Rail BEMU and British Rail Class 419. The Class 419, operational from 1959 to 2004 could run either on batteries or a third rail being used for short unelectrified section on quaysides.
Mark Carne of Network Rail in March 2017 stated: "The idea that you need to electrify an entire route is no longer necessarily the case. I think that where we have got hybrid trains, that opens up quite a lot of interesting opportunities for partial electrification." Carne also stated: "technology was advancing at such a pace that better reliability could be achieved without the construction of unsightly overhead cables." He added that developments in batteries are such that it might soon be cost-effective to swap diesel engines for battery or hybrid devices, therefore saving the vast cost of installing power lines above every section of track.
In 2015 a single Class 379 Electrostar, after electric lithium batteries were installed, began passenger operations on the Mayflower Line in Essex. The train can travel up to 60 miles (97 km) on energy stored in the batteries also recharging the batteries via the overhead-wires when on electrified sections of the line, at stations and via brake regeneration. The trial lasted one month from January to February. Network Rail refer to this prototype model and its possible future descendants as Independently Powered Electric Multiple Units (IPEMU).
A month after the trial in March 2015, the introduction of battery-powered trains was proposed by Network Rail for consideration for the fifteen station Wrexham to Bidston, Birkenhead, Borderlands Line. The aim is to connect the line onto the electric 3rd rail Merseyrail Wirral Line and extend into the underground section of Liverpool's city centre. The line is diesel train operated unable to progress into the electric 3rd rail underground Birkenhead and Liverpool Merseyrail tunnels. The diesel operated line terminates at Bidston railway station in Birkenhead, with passengers having to change trains at Bidston to proceed onto the Merseyrail electric network. Network Rail proposed using dual battery powered/3rd rail pickup rolling stock without full electrification of the 27 mile long line, providing a cheaper method of increasing connectivity into the electrified underground sections of the Birkenhead and Liverpool Merseyrail Wirral Line. If battery trains are introduced the Borderlands line will be incorporated into the Wirral Line becoming an urban metro line, in which trains only terminate at Wrexham running around the electrified Liverpool city centre underground tunnels in a loop.
On page 37 of the Liverpool City Region Combined Authority, Long Term Rail Strategy document of October 2017, it states that in 2020 a trial of new class 777 Merseyrail electric 3rd rail/battery trains will be undertaken in view to incorporate two stretches of unelectrified track onto the Merseyrail network. The sections are on the Wirral Line from Ellesmere Port to Helsby interchange station and the Northern Line from Ormskirk to Preston interchange station.
Vivarail have produced the Class 230 train which is converted from redundant London Underground D-Stock trains. The second demonstrator unit, the two-car 230002, has been fitted for battery power in contrast to the diesel-electric power of the earlier 230001 demonstrator unit. Vivarail have also developed a fast charger giving a 60 mile range. Power is provided by two lithium-ion batteries per car giving four per train, providing 106kWh, with an expected life of seven years for the batteries.
The Edison-Beach battery railcar was developed by Thomas Edison and Ralph H. Beach. The latter headed the Railway Storage Battery Car Company and the Electric Car & Locomotive Corp. Car No. 105 of the Alaska Railroad was an Edison-Beach car, and examples operated on the Central Vermont Railway running between Millers Falls, Northfield and West Townshend.  A notable feature of the Edison-Beach cars was the Beach drive system. Each wheel was mounted on ball bearings on a dead axle and was driven by an individual traction motor through gearing.
- Electric multiple unit
- Diesel multiple unit
- Battery locomotive
- Electric vehicle battery
- Flywheel energy storage
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