This is completely and utterly the preparation for another tax grab. No more and no less. But just to clarify some points above:
1. Gordon Brown first stimulated growth of, and then increased the tax on DERV and diesel cars because they allegedly were a bigger health hazard than petrol, while the EU took a different view and encouraged diesel.
2. It has been shown that trains emit far less pollutants than lorries or buses in proportion to the amount of people moved. Electrification deals with that one eventually anyway, despite recent wilfully bungled attempts.
Now with that out of the way I will point out that even now, the diesel engine has had far less development than the petrol engine, although the gap is narrowing. Economics and fuel consumption favour the diesel massively. If the demand for diesel falls, the refinery output has to be re-balanced with more catalytic cracking to make lighter hydrocarbons, so the fuel price rises, along with spurious by-products of the cracking process that have undesirable toxic effects.
I think that someone has already mentioned buses as a source of pollution. Clearly in London, many of the buses do not comply with the law, having very visible exhaust smoke. Why is that allowed to continue? It is quite a long term and major violation of the law, so why was not Boris or his predecessor Red Ken, up in court over it?
The other source of dangerous pollution in London is motorbikes. Stupid, archaic engines that can't idle properly, have poor low-down torque, extremely high fuel consumption in relation to the vehicle mass, and very peaky power at high revs, which is inappropriate but what the marketing spivs decide that the customer must have. Get behind a few of those in heavy traffic and you are immediately choking on unburned hydrocarbons, from unleaded fuel, due to the pathetically inefficient engines, some of which are highly carcinogenic.
The modern diesel cars in comparison are utterly insignificant, and the older ones scarcely significant. It is the usual political cycle of stimulate a demand, and once people are hooked, find a strong objection to justify taxing it. That is all it is. It is Gordon Brown all over again. Taxing diesel will be highly inflationary as it will increase the cost of everything, as it did last time.