For those asking what this means here is my attempt at a summary, hopefully not too patronising:
You can think of this as being about dust. Normal dust in a house varies from around 1 micrometre to 400 micrometres in size. A micrometre is 1000 times smaller than a millimetre, to give an idea a hair is about 50 micrometres wide.
The OP is measuring quantities of dust that is made up of particularly small particles, each less than 2.5 micrometres in size. These are so small they don't really settle on surfaces but tend to stay in the air and get breathed in. When they get breathed in they are so small that they move into your blood. Research has found that these cause health problems such as asthma, heart disease, lung problems etc. There is not a 'safe level' as such, the more of these particles there are the more likely they are to cause a problem.
The OPs measurements indicated that there is more of this dust in the underground than at street level, but that when in a new train the air conditioning reduces the amount very quickly. There is significantly more of this dust in diesel trains on the national rail network.
What can we do with this information as individuals? Not very much to be honest. But I do find the results interesting, and I would imagine that TfL and train companies carry out their own monitoring that they use to try and buy cleaner trains going forwards.