TAKE a car, add an electric motor and some batteries. Build in a special gearbox which combines the output of the motor with that of an internal-combustion engine. Now all you need is a sophisticated electronic control system, and the result is a hybrid car like the Toyota Prius. It saves fuel, because when the driver brakes the car stores the lost kinetic energy in the battery, and when the driver hits the accelerator the electric motor kicks in, using that stored energy. But it took Toyota 13 years to convince customers that the Prius principle is the technology of the future.

Now, it seems, it has convinced builders of railway engines, too. Like carmakers, they are trying to combine internal-combustion engines and electric motors to save fuel. “Development is so advanced now, that we can use it, too,” says Peter Rieger of MTU, a German engine-builder. The main difference is that whereas the carmakers were able to design new vehicles around their new technology, locomotive-builders have had to construct their hybrid engines as packages that can replace the original motors of existing machines.

As with cars, Japan leads the way. After a three-year trial JR East, a privately owned railway company, announced that it would introduce ten diesel-battery hybrid railcars. This would be the first fleet of hybrid diesel multiple units. The system is expected to reduce fuel consumption by 10% and to be much quieter than the current arrangement. But, unlike the European car industry, the European rail industry is not far behind Japan. MTU, for example, has developed a hybrid engine-gear package which can simply replace a conventional motor. Its engineers added an electric motor and a special gearbox to combine the output of both engines. All the components are built into a frame, which can be put into the space the old engine occupied.

Other manufacturers have similar ideas. Voith Turbo, a German company known for its diesel-hydraulic transmissions, has linked up with MAN, an engine-builder, to develop a hybrid power-pack that could replace conventional drives. And, in America, General Electric (GE) is also working on a hybrid engine. This will be equipped with a large number of batteries that store the kinetic energy otherwise dispersed when the locomotive is stopping long freight trains. That will reduce fuel consumption by about 15%.

Which is not enough for the engineers at the German branch of Alstom. They have converted an old heavy shunter into a hybrid, slashing its fuel consumption by 40-50%, according to Klaus Hiller, who is in charge of the project. To do so, the company turned the Prius principle upside down. The main motive power comes from two large batteries. The diesel is merely a backup.

The problem of hybrid trains, however, is the same as that of hybrid cars: the batteries add weight. Admittedly, this is not quite such a hardship in locomotives. They need a certain amount of weight in order to keep a tight grip on the rails. But railways are supposed to carry people, not batteries, says Mr Hiller. So those batteries have to weigh as little as possible.

One way round this is to use lithium-ion batteries, which are also favoured by those making the latest generation of electric cars. These batteries, however, have a short lifespan. At the moment, they last about five to seven years, compared with a train's average life of 30 years.

There is also the question of whether lithium-ion batteries are tough enough for the world of railways. According to Mr Hiller, his shunter has to withstand decelerations of 5g (ie, five times the force of the Earth's gravitational field). It is not clear that lithium-ion cells can withstand such forces for long. Alstom has therefore turned to tried-and-trusted nickel-cadmium ones instead, and GE is testing sodium and iron-chloride batteries.

One way or another, however, the problem needs to be solved soon. Emission regulations for railways have recently been tightened in Europe and will become even stricter in the future. From 2012 diesel trains will have to reduce their emissions of nitrogen oxides by 39% and of soot by 88%. Hybrids may be the only way to meet these requirements. For the Prius principle, then, the future looks bright.