It all comes down to efficiency; electric motors are far more efficient than internal combustion engines when it comes to creating mechanical energy, and this is particularly relevant with trains.
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Every battery locomotive that replaces a diesel will reduce carbon dioxide emissions by 3,000 tons per year, Wabtec estimates. But it is unlikely they can quickly replace diesel-powered trains.
Some locomotives collect electricity from overhead cables, while others take power from a third “live” rail on the track. It is very expensive to build the lines or rails that carry the electric current, but electric locomotives are cleaner, quieter, faster, and more reliable than steam or diesel engines.
The Association of American Railroads claims 220000 km. Capital costs for electrifying something that large would be just a bit high, and with continuing subsidies for oil the cost savings simply aren't there. Additionally, the vast majority of the US rail network is privately owned and operated by dozens of railroads.
A train engine requires about a hundred litres of fuel to get it started. So it wouldn't be economical if the engine is stopped and started frequently. This apart, if the engine is stopped, the moving parts' lubrication will also come to a halt.
Electric trains are more quiet, and emit less sound than diesel engines. Electric locomotives don't have reciprocating parts so they are more easy to use on the track. They also require less maintenance.
Certain electric traction systems provide Regenerative brakes. The energy generated by the train is converted into electricity and return it to the power system that is available to be used by other trains, or to the utility grid in general.
Diesel locomotive engines are a major contributor to air pollution. The culprits are NOx SO2. Both are readily produced by diesel locomotives and both cause an array of health and environmental problems.
If we take an overall view of the transport sector, 71% of transportation related carbon emissions come from road users, whereas only 1.8% of emissions stem from rail travel. So in absolute terms, trains are responsible for a lot less emissions than cars.
The diesel-electric locomotive has a thermal efficiency (percent of useful work derived from a unit of fuel) of 20–25%. Since a gallon of fuel oil contains approximately 130,000 Btu of energy, only 20–25% of this would be available to produce work, that is, pull the train.
The decision not to procure new diesel trains had already been made in 2020. As soon as trains have reached the end of their economic life, they are now to be replaced with battery trains. Trains that are theoretically still operational are to be used on non-electrified lines during the transition.
Disadvantages of electric traction include: high capital costs that may be uneconomic on lightly trafficked routes, a relative lack of flexibility (since electric trains need third rails or overhead wires), and a vulnerability to power interruptions.