For most rail services, there is only one decarbonisation solution. As Transport Scotland's Bill Reeve is often heard to say, "In Scotland, we spell decarbonisation E-L-E-C-T-R-I-F-Y."

There are sound reasons for this quip. Railway electrification is the only proven zero-carbon high-powered transport technology. Electric trains use electricity as it is generated and return it to the grid when braking. The power of electric trains is only limited by the current that can be passed from wire to pantograph and the thermal capacity of their traction motors.

In contrast, the power of self-powered trains is limited by their heavy on-board power plant. They also require on-board energy storage which involves inefficient energy conversions. This is not an issue for electric trains which are more efficient, powerful, and lighter than self-powered traction as well as being cheaper to buy, operate and maintain.

Electrification therefore offers significant financial and operational benefits. Electric traction offers the high acceleration needed for high-capacity metro services and heavy freight trains with acceptable performance on a mixed traffic railway and is the only way of powering very high-speed trains. Its disadvantage is that it is expensive but, in some cases, much more than it need be. Moreover, decision makers focus on the cost of electrification rather than its benefits. As a result, the UK has comparatively little electrification and so has one of the world's worst railway carbon records. It probably operates the world's most intensive diesel inter-city service and 97% of the energy used by rail freight is from diesel.

Scotland gets it

Unfortunately, there is little sign that the UK Government understands that a rolling electrification programme is needed if it is to decarbonise its railways. As an example, the response to a Parliamentary question asking why the new East-West rail line is not being electrified as it is built was that this enables a wider range of green energy technologies to replace diesel trains to be explored.

In contrast, Scotland is the only part of the UK that is actually implementing a plan to decarbonise its railways with a rolling electrification programme. This is being done, not just because decarbonisation is the right thing to do, but because it provides a more cost-effective, reliable, and higher performing railway. As a result, more passenger and freight customers will be attracted to rail as required by the Scottish Government's overall decarbonisation plan. Scotland's Rail Services Decarbonisation Action Plan envisages the elimination of diesel passenger trains by 2035. Thus, by then alternative zero-carbon trains should be operating on the Far North Line. By 2045 the plan is that all Scottish lines will be electrified apart from: Girvan to Stranraer; Craigendoran to Oban and Mallaig, Dingwall to Kyle of Lochalsh and Tain to Thurso and Wick. Thus, interestingly Transport Scotland considers that, there is likely to be a case for electrification from Inverness to Tain. Yet with less frequent services, the remaining 74% of the Far North Line will require a self-powered alternative to diesel traction.

Weaning transport off fossil fuels

Since 1990 the UK has reduced its carbon emissions by 44%. Thus it could be thought that the UK is on target to achieve its target of net-zero carbon emissions by 2050, yet this is not the case. Whilst there has been a 66% reduction in emissions from power generation, there has only been a respective 5% and 15% reduction in transport and residential emissions which between them account for 43% of UK emissions as both these sectors are heavily dependent on gas and liquid fossil fuels. The issue is that nothing else comes close to the amount of energy that can be stored in easily transportable fossil fuels unless a nuclear-powered train is feasible.

In respect of energy density, hydrogen is the closest practically available substance to petroleum. This can be stored as a compressed gas, as a liquid when super-cooled, or in ammonia. Liquid hydrogen fuel has been used for space rockets and is being considered by Airbus for zero-carbon planes. Ammonia is being considered for zero-carbon ships. The table below compares energy storage of hydrogen and batteries with that of diesel fuel by volume and weight.

Although compressed hydrogen at 350 bar has only one-twelfth of the volumetric energy density of diesel fuel, this is the highest energy density of any practical alternative and is 72% greater than a battery. However, by 2035, battery energy density could be much closer to that of hydrogen.

For passenger trains, hydrogen's low energy density might result in a loss of passenger space as is the case with the Hydroflex train shown in Glasgow during COP26. However, this is not a problem for Alstom's proposal to build a new hydrogen train which will accommodate roof-mounted tanks within the UK loading gauge.

A crucial point is that the ability of fossil fuels to store so much energy and their relatively low cost of extraction and refining makes them much cheaper than zero-carbon alternatives. Hence zero-carbon alternatives are not likely to be widely used unless fossil fuels are subject to a carbon tax. Whether this is a realistic aspiration remains to be seen.

Another possible zero-carbon alternative is using diesel engines powered by sustainable fuels. However, truly sustainable fuels are expensive and will be a limited resource. The Committee for Climate Change (CCC) suggests that sustainable biomass could replace a maximum of around 10% of fossil fuel energy by 2050. Synthetic sustainable fuels are an option but, as they use hydrogen as a feedstock, they will be more expensive than hydrogen.

Hydrogen economy

Worldwide production of hydrogen is currently 80 million tonnes per annum with almost all production by reforming methane. Emissions from such hydrogen are only slightly less than fossil fuels. Zero-carbon hydrogen needs to be produced by electrolysis powered by net-zero electricity.

Hydrogen produced by electrolysis is more expensive than that produced by reforming. Yet the cost of hydrogen produced by electrolysis is essentially the cost of the kit (i.e. wind turbines and electrolysers) which, unlike the price of fossil fuels, is generally predicable. For this reason, a recent order for hydrogen trains was able to include 30 years supply of hydrogen. Businesses like price certainty.

The CCC's net-zero report shows that, if the net-zero target is to be met, the UK needs a large hydrogen economy. Speaking at COP26's Hydrogen Transition Summit, Scottish Government Minister, Michael Matheson, also stressed that Scotland considers hydrogen to be a crucial part of a low energy future. He felt that "wind was the new oil" as Scotland's offshore wind turbines could be used to produce cheap hydrogen for export. Moreover, the well-established petrochemical sector has the skills to diversify into a hydrogen economy.

This summit also considered the use of hydrogen for domestic heating. For this, the UK's Department for Business, Energy, and Industrial Strategy (BEIS) set up a £25 million Hy4heat initiative in 2017. This has developed a range of hydrogen appliances and produced a safety assessment, that has been independently reviewed by the Health and Safety Executive, which concluded that hydrogen could be as safe as natural gas in the home. As part of this initiative, the world's first hydrogen neighbourhood in Levenmouth will see up to 300 homes heated by hydrogen in 2023.

The summit indicated that by 2050, the hydrogen economy could be worth hundreds of millions of pounds. Key themes during the event were the synergy between hydrogen and renewable power generation and the importance of creating a strong end-use market for which a global carbon tax on fossil fuels was required. As previously mentioned, whether this is realistic remains to be seen.

A full report on this summit is available online: "Rail Engineer: Hydrogen fuel of the future?"

Batteries

Significant advances in battery technology now make battery powered trains a realistic option in some situations. Both the Welsh and Scottish Governments are respectively developing permanent and transitional partial electrification options that will use battery powered EMUs that are charged whilst under the wires. Yet, as shown by the table, by volume, a traction battery can only store one-twentieth of the amount of energy of a diesel tank. Hence, battery powered trains have a range between charges of the order of tens of kilometres.

Rapid charging technologies offer an opportunity to extend this range if it is acceptable to stop a train for, say, 10 minutes every 100 kilometres. Whilst there is no doubt that improved batteries will be developed, as shown by the table, it is unlikely that this will dramatically increase their storage capacity.

Both the monetary and carbon cost of batteries also needs to be considered. Most analysts consider that battery costs will continue to drop. Yet there are currently 1.4 billion cars and vans in the world of which less than 1% are electric vehicles. What happens to battery prices if and when a substantial proportion of these vehicles become electric is an interesting question.

An indication of battery carbon cost is that, with current grid CO₂ emissions, a small car will take around three years to save the CO₂ emitted making its battery. Larger luxury cars will take up to six years to pay back that carbon.

Future Far North trains

For almost all the rail network, rail decarbonisation requires electric trains. They are a future-proofed solution as no amount of innovation can produce a zero-carbon self-powered vehicle that is more efficient that one that uses electricity as it is generated.

For those few lines, such as the Far North, where electrification is not appropriate, zero-carbon options are sustainable fuels, batteries, and hydrogen. It is unlikely that any new solutions will emerge at this stage. With good reason, Scotland's rail decarbonisation plan does not specify the type of alternative traction for non-electrified lines as no-one can know what the best solution will be in 2035 so it is best to keep options open. Hence the development of Scotland's hydrogen train at Bo'ness is a worthwhile initiative, particularly as this could be part of the Scotland's developing hydrogen economy.

My informed guess is that by 2035 the Far North and Kyle Lines will be operated by hydrogen trains. This is because sustainable fuels will be rare and expensive. Moreover, to provide the required range, battery development would need to exceed that predicted. There are issues of embodied carbon (the CO₂ emitted during manufacture) and cost, should there be an exponential increase in worldwide demand.

Hydrogen is currently the only net-zero option that provides the required range. It should also be part of the emerging hydrogen economy. This key factor is not considered by those who dismiss hydrogen trains. Yet a large-scale hydrogen economy would need a global fossil fuel carbon tax. Whether the world wishes to pay such a tax to ensure net-zero carbon emissions remains to be seen. If not, there will still be some demand for hydrogen as it is essentially a fixed-cost fuel and also offers the grid balancing required by renewable power.

I hope the above explains the vague "should probably" in the title of this article, especially as predictions can take some time to be realised. For example, in 1874, Jules Verne wrote of a world where "Water will one day be employed as fuel, that hydrogen and oxygen which constitute it will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable." It will be interesting to see if, 161 years later, the Far North Line will prove him right.