When he was a little boy Pandora was much delighted by the story of the Magic Porridge Pot which, through an agency never made quite clear, poured forth unlimited amounts of porridge. Now that he is older Pandora can see that this story needs to be taken with more than a pinch of salt. Would that the guiding minds in the British railway were so alert to the tricky bits of delivering an infinite quantity of good things without working out quite how. A year ago Pandora looked at the different energy densities of various competing fuels: electricity, diesel, battery and hydrogen. The inescapable conclusion - purely from an energy delivery standpoint, and ignoring cost - was, and remains, that electricity and diesel are vastly more efficient than the new trendy kinds. Diesel will soon be taboo, leaving electricity as the only source of powering the bulk of trains - long-distance expresses and all freight. Shortish lines can get by with some hybrid solution, but these form a small percentage of the total track miles. None of this is new to Pandora's readers.

This is the easy bit: make sure that an unlimited amount of electricity appears (and while we're at it, make sure that an unlimited supply of all the fancy new stuff needed for batteries appears as well. The hydrogen is easy - you just stick some electricity into a bowl of water and...but that merely adds to the amount of electricity needed, so we need an even bigger amount of electricity than we first thought of). Let's forget all the rare earths and metals sourced from unfriendly places, as Pandora allows himself to get worried by only one thing at a time.

Add up all the electricity generated. X% will be used in industrial processes; Y% used by people in their homes; Z% used by existing transport concerns (mainly trains, but also some buses, trams and goods vehicles). Pandora does not know the values for X, Y and Z, but thinks it unlikely that X and Y will alter much as time passes, though neither is likely to fall as other fuels for industry and domestic purposes will be reduced. Z will increase greatly. This can have only one result: all the electricity currently generated will be insufficient. And all that is before we consider the unknown unknown: the wholly extra Q% which will be required to power all those electric cars.

Here is the rock: the need to generate at least a further Z% within the next 15 to 20 years. The hard place is that there is no Magic Porridge Pot other than the One Whose Name Cannot Be Mentioned. Covering the land with windmills or solar panels will not do the job as the amount of land take which is likely to be acceptable is nothing like enough, and each introduces the additional problem of storage for when the wind doesn't blow or the sun doesn't shine. Pandora would not wish to see the tabloid newspapers with headlines blaming the lack of trains on the wrong kind of sunshine.

There's no getting away from it, is there? When faced with two choices, neither of them agreeable, the only solution is to accept the less (and here it's necessary to insert a negative adjective: disagreeable? dangerous? unpopular?) one. The clever thing will be to find a positive adjective instead. If we are to carry out the policy of decarbonising rail transport we must build more nuclear power stations. Plural. And soon. For they are the nearest things to a magic porridge pot.