Oxford Ragwort (Senecio squalidus)
This is not the Common Ragwort seen in meadows in the autumn, it is a totally different flower although superficially similar.
It was an imported species brought in to Oxford, possibly the University as part of a research programme, many years ago. I did hear the story some years ago and no can't remember it!
Like Common Ragwort these flowers turn into small dandelion type seed heads which are dispersed by wind. They were flowering near the main railway line in Oxford and the seeds were carried along the line in the slip stream of trains and now they are seen across the entire rail network.
They originate from volcanic areas where they thrive on thin, dry soils so the chippings on railway lines is well suited to their needs.