Here, in the US, untaxed diesel is dyed red. The authorities typically don't bother checking passenger vehicles but commercial trucks are routinely checked. If the fuel is darker than some arbitrary limit, there are severe fines imposed.

Is "off-road" diesel actually formulated any differently? My understanding is that it's taxed differently because it's only legal to use it in machines that aren't road-legal (farm & construction equipment, for instance), since much of fuel taxes goes toward road maintenance. I thought that the only real different is the presence/absence of a dye used to prove that you're using the correct type.

As I understand it, it's legal to use it in other vehicles but only if those vehicles are used off of the public roadways. For example, if you have a large site and must move material internally to that site, you can use untaxed diesel while the vehicles are moving material on site.

Apparently they used to have higher sulfur but as of 12/2010 are all supposed to be Ultra Low Sulfur, although diesels will run on heating oil.

I have heard from someone who worked with a diesel fuel distributor that most of the "off-road" diesel in the US is exactly the same as over-the-road diesel and that the only difference is the tax paid and the colorant added.

Build a small box with a piece of an unused HEPA filter for a vacuum cleaner, sandwiched between two small computer case fans in push/pull config, add some batteries and a on/off switch. Take that on the train and leave it whirring. Store it in a ziploc bag that you open after getting on the train and close before leaving again.

You then have to send that sample off to a lab, since DYI-ing an elementary carbon analysis looks bloody complicated and expensive.

The device for measuring particulate concentration is called an aethalometer: https://en.wikipedia.org/wiki/Aethalometer