Sometimes it is worth considering first principles. All transport systems require the conversion of chemical or atomic or potential energy into mechanical energy. The preferred method of doing this is to carry out the initial conversion at a central location such as a power station. This has been realised since almost the beginning of railways, first with the use of rope traction, then with the atmospheric railways in the 1840s, and eventually with the development of electric traction.
But electric traction means that the railway must be provided with a fixed supply of electricity, usually in the form of an overhead contact cable or conductor rail. This is a mature technology and the preferred option where traffic densities are high and the cost of installation can be justified. On routes where traffic volumes are low, the conversion of chemical energy to mechanical energy must take place in a prime-mover vehicle of some kind. The chemical energy can be converted into electrical energy and stored temporarily in batteries but the energy density is low, though the technology improves all the time. The most energy-dense substances at normal temperatures are hydrocarbons with about 8 carbon atoms per molecule, which is why they are the most desirable of fuels. Biomass in its various forms is of low energy density and large volumes are needed.
Internal and external combustion
Hydrocarbon fuels can conveniently be converted into mechanical energy in an internal combustion machine. In smaller sizes these are mass produced and inexpensive and have the advantage of being compact. Internal combustion gives high efficiency of conversion but the rate of combustion of the fuel must at all times precisely match the output of mechanical power, consequently the machine must be sized for the maximum output. Hybrid devices are possible in which the mismatch between the rate of conversion of chemical energy and the power requirement is evened-out through the use of an energy storage system such as a flywheel or electric battery. Internal combustion machines have a few inherent disadvantages. The fuel must be carefully formulated. They are inherently noisy and give rise to vibration. Combustion is incomplete and results in the production of undesirable products such as particulates and nitrogen oxides. They have low power and low torque when running at low speeds, which means that a power transmission system is needed, adding complexity and reducing efficiency. Larger internal combustion machines are difficult to start, and consequently tend to be left running when not actually in use.
External combustion machines separate the combustion process through the introduction of an intermediate stage in which energy is stored until required for use, usually in the form of a hot compressed gas. This enables better control of the combustion process, with the fuel being fully oxidised, thereby preventing the emission of particulates. The combustion system can be sized to match the energy output requirement, averaged out over an appropriate period. The conversion of heat to mechanical energy then takes place in a separate device, and only this needs to be sized for maximum output. Such machines typically deliver high torque at low speeds, and no transmission system is needed, significantly reducing initial costs, transmission losses and maintenance requirements. In theory, any material that will burn can be used as fuel.
At any point in time, circumstances and the state of the technology will affect the relative advantages of the different options. When investment decisions are being made, it is essential to evaluate all the possibilities.