Employing regenerative braking in trains can lead to substantial CO2 emission reductions, especially when applied to full stop service commuter trains (8 – 17%) and to very dense suburban network trains (~ 30%). Regenerative braking applied to freight trains can also lead to CO2 emission reductions, albeit considerably lower than for full stop service trains (~5%). When regenerative braking is employed, the current in the electric motors is reversed, slowing down the train. At the same time, the electro motors generate electricity to be returned to the power distribution system. Regenerative breaking is a mature technology. It can be more easily applied to AC powered trains than to DC powered systems. In DC powered railway systems usually higher investment costs are needed.
A conventional electric train braking system uses dynamic braking, where the kinetic energy of the train is dissipated as waste, mainly in the form of heat. When regenerative braking is employed, the current in the electric motors is reversed, slowing down the train. (UIC, 2003) At the same time, the electro motors generate electricity to be returned to the power distribution system. This generated electricity can be used to power other trains within the network or can be used to offset power demands of other loads such as lighting in stations. However, the power recovered via regenerative braking can only be used if that power is simultaneously being drawn somewhere else. In general no power is recovered when the overhead power is out.
The two main motivations to employ regenerative breaking are energy savings and reduced wear of mechanical brakes. The technique of regenerative braking is most effective in full stop passenger trains and subway trains (metro), because they stop often enough to make recovery worthwhile. Conventional freight trains only have a limited potential to recover power with the help of regenerative braking. (UIC,2002a) This is due to the high average weight of freight trains and the fact that only the locomotive axles are powered. The main share of braking is done by the mechanical brakes located on the freight cars, and only a small share originates from the locomotive itself.
Various types of trains can be equipped with regenerative braking: electric trains, hybrid diesel locomotives and subway trains
The more frequently a train stops, the more it can benefit from regenerative breaking. Therefore the technique is especially valuable for commuter trains and subways, which both stop frequently.
Electric railway systems can be either DC or AC powered. It is much easier to implement regenerative breaking for AC powered systems. For DC powered systems, there are two main barriers: (1) Most DC powered systems use relatively low voltages and (2) often the generated electricity cannot be fed back into the public electricity grid. In very dense suburban DC powered networks, however, regenerative breaking can be an effective way to reduce the electricity demand. In all other cases, the effectiveness of regenerative braking is rather low but may be enhanced by technological upgrades of vehicles and/or substations. These upgrades are associated with relatively high investment costs.
Railway systems working with AC power can implement regenerative braking with almost no additional costs. Also the implementation of regenerative braking in diesel powered locomotives poses no obstacle. Virtually all locomotives are diesel-electric, so the capacity to do regenerative braking is available.
Regenerative braking is a mature technology. Within Europe, there is still a considerable difference between countries in the share of rolling stock that is equipped with regenerative braking, but the share is relatively high already. Regenerative breaking is relatively standard in new trains.
It is also used in major new high-speed trains. For example the new N700 series of the Shinkansen in Japan, which became operational in February 2009, uses regenerative braking. However, friction brakes are still needed as backup in the case that the regenerative brakes fail. It is possible to use regenerative braking on these high speed trains because most cars have their own electric motors, this is in contrast to trains in which only the locomotive has electric motors. The fourth generation TGVs in France, which are expected to be commissioned in 2010, will also be equipped with regenerative brakes, as will the German ICE 3 trains which are to be commissioned in 2012.
Contribution by the use of regenerative breaking to socio-economic development is expected to be low. The Delhi Metro CDM project (DMRC, 2007) argues for marginally improved local employment in the operation and maintenance of the trains, but does not go into details on this.
The effects of regenerative braking on air quality depend mainly on the way the electricity is produced. In general, the introduction of regenerative braking on electric trains and subway trains will have no direct effect on the local air quality. However, lowering the electricity demand will lower the emission of air pollutants, like NOx, SO2 and particulate matter in power generation, if power generation is based on fossil fuels.
For diesel powered locomotives, hybridization can have a positive direct effect on air quality, depending on the usage pattern. Locomotives used solely on a marshalling yard can achieve very high reductions in emissions, due to frequent need for braking. However, the reduction in local air pollution will be limited when the locomotive is used in long-haul freight trains.
The following lists some examples of energy savings in different types of trains using regenerative braking. The final effect of regenerative braking on CO2 emissions depends on how the trains are employed, and on the generation mix of the electricity used.
Electric trains can recover 8 to 17% of electricity depending on whether the trains is used as long distance train or a full stop commuter train. In the United States, Amtrak introduced Acela Express high speed trains and other new and remanufactured electric locomotives in 2006. These trains use regenerative braking systems and have allowed Amtrak to reduce energy consumption by 8%, while in the UK Pendolino trains return up to 17% to the grid.
At the beginning of 2009, GE division Locomotive was designing a hybrid diesel locomotive for Indian Railways to capture energy dissipated during braking and store it in batteries to be used later. The new locomotive should reduce the fuel consumption by 15 percent and NOx and fine particle emissions even by 50 percent. It is unclear whether this fuel consumption reduction can be achieved when pulling many freight cars. (UIC, 2003)
The use of regenerative braking in the New Delhi's metro cuts back on energy use of the trains by about 30%. (Ritch, 2009)
For high speed trains, the reduction in electricity use by employing regenerative breaking is in general a bit lower. The energy savings of the N700 series of the Shinkansen are estimated to be about 4.5%. (UIC,2009)
In general the highest amount of energy can be recovered on full stop service commuter trains and subway trains.
Few explicit cost estimates for the incremental investment costs of a regenerative breaking system were found. In the case of the Delhi Metro Rail Corporation CDM project which installed 25 kV AC powered coaches, the total additional investment costs compared to using 750 V DC rheostatic braking systems were reported to be in the order of 114mio Indian Rupees for 280 coaches (about 9200 USD per coach). (DMRC, 2007)
Cost benefits from the use of regenerative breaking stem from reduced energy costs and lower maintenance costs of the mechanical brakes. The full stop commuter services at Birmingham and Manchester in the UK are for example able to use regenerative braking. With regenerative braking being enabled, their disc brake pad life was around 18 months. When the electric braking was switched off, the pad life reduced to 18 days. (Ford, 2007) As a consequence of less needs for replacement, regenerative breaking also reduces the down-time of the train.
[Part of this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group. ]
Project developers of projects using regenerative breaking in trains under the CDM mainly apply the following CDM methdology: AMS-III.C.: Emission reductions by electric and hybrid vehicles 
As of March 2011, there are 3 projects using regenerative breaking in trains in the CDM pipeline, out of which 1 project is registered and CERs have been issued for this proejct. The project deploys coaches with regenerative breaking in the Delhi Metro. The project was submitted to the CDM Executive Board in the beginning of 2007 (Ritch, 2009; DMRC, 2007).
Delhi Metro Rail Corporation (DMRC) (2007) Installation of Low Green House Gases (GHG) emitting rolling stock cars in metro system. CDM project design document. Available at http://cdm.unfccc.int/Projects/Validation/DB/VD46D5DZB8DYZZD5RDXA9RVFWBR3RQ/view.html 
Ford (2007): Regenerative braking boosts green credentials. In Railway Gazette (online), available at http://www.railwaygazette.com/news/single-view/view//regenerative-braking-boosts-green-credentials.html 
Ritch, E. (2009): Delhi Metro earns carbon credits for regenerative braking system. Cleantech Group News. Available at http://cleantech.com/news/4201/delhi-metro-earns-carbon-credits-re 
UIC (2002): Regenerative braking in DC systems, available at http://www.railway-energy.org/static/Regenerative_braking_in_DC_systems_103.php 
UIC (2002a): Regenerative braking in freight trains. Available at http://www.railway-energy.org/static/Regenerative_braking_in_freight_trains_43.php 
UIC (2003). Energy efficiency technologies for railways. International Union of Railways. http://www.railway-energy.org/ 
UIC (2009) The energy efficiency days 2009. http://www.energy-efficiency-days.org/ 
UK Department for Transport (2007). Low carbon transport innovation strategy. Department of Transport, 2007. available at http://www.dft.gov.uk/pgr/scienceresearch/technology/lctis/lowcarbontis?page=9