In the coming months passengers on the Highland Chieftain running daily between Kings Cross and Inverness will have the pleasure of travelling in a bi-mode Class 800 Azuma. Nostalgia lovers who wish to continue to enjoy the comfort of an HST over the long demanding climbs of the HML north of Perth will have to use ScotRail services - pure diesel. Timing comparisons will be interesting as the "pocket rocket" MacHSTs will be 2+4 or 2+5 formation with a much better power/weight ratio than VTEC's 2+9 formation for the HC nowadays. Why does any of this matter?

The rail minister south of the Border has ruled that he "wishes to see" (which is, of course, not the same thing as will legislate to ensure that) "all diesel-only trains off the track by 2040". At the same time the Secretary of State is busy not electrifying anything, fearful lest the GWR contagion spread. This is the wrong response. That NR got the planning and execution hopelessly wrong on the GWR does not mean either that electrification is far too expensive or, more significantly, that NR has not learnt some painful lessons whose benefit would only be felt in future electrification projects. See bath-water, see baby.

If we as a nation are seriously to reduce the carbon footprint of the railway it can only be achieved meaningfully by vastly more electrification. The Netherlands, admittedly rather smaller than the UK and benefiting from having to rebuild pretty much the whole network after 1945, has 76% of its network electrified; GB a mere 42%.

But the SoS has set his face against it. There are "better" ways, the bi-mode being one of them. A bi-mode is not a system of propulsion which would have been designed by anyone starting from a clean sheet. It is a fudge. While operating under overhead wires it is dragging heavy unused diesel engines together with the fuel; while it is operating away from the wires it is dragging an electric motor. The benefit of a bi-mode is that it can operate anywhere, but wherever it does so it will operate less efficiently than a vehicle designed for operating under, or away from, the wires. Class 800s will be expected to remain in service for at least 40 years, so we are stuck with fudge for a long time. The real killer, of course, is that having expensive (very expensive) bits of kit which will last for 40 years and won't need another inch of wiring being installed significantly weakens the case for doing any more electrification. They'll still puff out diesel exhaust after 2040, but the fact that there's an electric motor (heavy, and not producing power) being dragged along makes it all right. It's not just the laws of physics which are being ignored here. There's common sense too. Under the wires an 800 develops 11.2kW/tonne; using diesel it's only 6.9kW/tonne. Guess where the hills are. This is why Transport Scotland must continue its excellent policy of electrifying 100 single track kilometres of track each year into CP6 (from which any electrification is absent, apart from maybe getting to Perth) and CP7. The HML would be the natural place to start.

The industry has been tasked by the rail minister south of the Border to provide a "vision" for how it plans to decarbonise "by the autumn". Pandora would love to see that report, particularly if it said "electrify all main lines and such joiny-uppy bits that will enable 90% of freight services and 95% of all passengers services to be electrically-powered by 2030". None of this 2040 long-term nonsense.

It won't say that, of course. It will bring to the table some exotica, and this is where the laws of physics really come into play. Battery power and hydrogen power are under development, and trains using them are already in service in Europe. Each has its place, but neither will be any use for long-distance fast passenger work, nor for freight. Batteries take longer to charge than it takes to refill a diesel tank, so battery power is only any good where there is an out-and-back journey of no more than say 40km, or where any longer journey is under the wires, allowing charging while on the move. Battery technology is developing very rapidly, and no doubt the range will be significantly further in a few years' time. However by their nature batteries have low energy density. An ordinary lead/acid battery can deliver around 0.56MJ/litre, while a lithium ion battery reaches over 2.6MJ/l. Neither is high enough to produce nippy acceleration, nor powerful enough to start a heavy train. This battery power will only be used on relatively short-distance lightly-loaded passenger services. The Wick-Thurso shuttle would be ideal, provided charging could be done at each end, and it took no more than say 20 minutes. Scotland is, or could be, well-provided with such short journeys. Borders Railway, Levenmouth, some of the Glasgow outer suburban routes still served by DMUs.

Hydrogen is seen as a magic solution, mainly because it scores extremely well on the laws of chemistry (ie. no pollution). Its energy density, at 2.7MJ/l, matches that of a Li-ion battery. The laws of physics rather spoil things, however, because the energy density of diesel is 35.8MJ/l. Someone somewhere must make an intelligent decision about the weight which should be accorded to a pollution-free fuel against one which actually generates a decent level of grunt. One hopes that this person is possessed of Physics A-level. Hydrogen will, like battery, be ideal for some rail needs. While the technology is being developed it isn't easy to see what those might be, but it's fairly clear that freight haulage won't be one of them. An energy density that low would suggest that, like battery-operated rolling stock, hydrogen power will find it difficult to move heavy trains easily. Once they're moving quickly, however, hydrogen would easily be able to keep them going at high speed since relatively little power is needed for this. What sort of trains will need to accelerate infrequently, to go very fast for a long time with little need to decelerate for stations or adverse signals? Have the words high speed and hydrogen been seen in the same sentence before? Probably not, because HS2 will be all-electric, and electric haulage beats hydrogen. So hydrogen will likely fill the same niches as battery, albeit probably more efficiently and with less need for frequent charging infrastructure.

Were this article to be revisited in 10 years' time the technology would be changed, probably beyond all recognition. But the laws of physics would not.