I've talked before about bi-mode trains. Most 'diesel' trains are actually diesel-electric. A diesel generator charges batteries (in the old days, lead-acid batteries, and for all I know, that may still be the case) which then drive the electric motors which turn the wheels. Why such an apparently complicated arrangement? It's because electric motors have a higher torque than diesel motors, and can therefore accelerate away from stations more quickly. A bi-mode train is one that can, when it's possible, directly use electricity from an overhead wire or third rail to drive the engines, obviating the need for diesel; or can use the diesel engine when there is no electricity source.
There is also a battery-electric train, which is not quite the same thing. This charges up its batteries whenever there is external electricity, then uses the stored power in its batteries when the wire ends. This option, won't work, however, in places where the distance between population centres is large, such as Australia, because batteries are not yet energy-dense enough to cover long distances.
Electric traction is the "best" for trains in terms of operating cost. But the capital and maintenance costs are high, and so expanding electrification to the routes where there are only a few trains a day is not cost-effective.
In NSW, which is between California and Texas is size, the rail network within a couple of hundred kilometres around Sydney is electrified, but the majority (in terms of track, not population) of the state is served by diesel. The NSW State government has started replacing aging diesel trains with bi-mode trains, which can switch between diesel(-electric) in the countryside to electric in the city. Despite a minority of the network being electrified, the reduction in emissions is still significant.
In the UK, Hitachi isn't just building battery-electric trains, it's also developed bi-mode trains, as this video below shows.
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