This was taken at Wuerzburg, Germany. It says more or less that In this house, W.C. Roentgen discovered in the year of 1895 the X-rays. Roentgen's Nobel Prize medal and many memorabilia like letters, notebooks and pieces of equipment can still be seen in the Physics Institute of the Wuerzburg University. I should have taken some pictures of that as well. Well, the above building is actually close to the city centre, far from the main campus of the university where the physics building is located, but it is still part of the university anyway. You can find more about Roentgen's life here in this Wikipedia article.
And then I was coming back from a day out in a historical house with some friends here in the UK when we decided to stop in these small town half way between there and Birmingham. Imagine my surprise when after we started walking around I saw the above plaque. No need to say that my friends didn't get so excited as myself with the finding. Here is the bio of Willian Henry (the father) and William Lawrence (the son). For those who may not know, X-ray crystallography is probably the most important method to probe the structure of not only crystals but many kinds of proteins and it was through X-ray measurements that Rosalind Franklin collected the data that allowed for the discovery of the DNA double-helix structure.
Yes, I know it's not exactly a trail... but still, I cannot resist to take these pictures when I pass through those places. By the way, if you have any additional suggestion for the X-ray Trail, that would be interesting. Maybe I can even spend some time to collect them (if there is any) in a document. :)