What happens if the train company passes my information over the debt collection agency, who then charges interest for as long as the fine is outstanding then pursues it once it reaches a figure worth chasing you for? Is that a possibility.

Nope I have no plans to travel to Germany again in the near future, I'll write them a letter and see where I get. I guess I'll continue to receive post until I sort the issue out because it appears to be all automated.

In Germany a few months back I was caught travelling without a valid underground ticket, I gave them my id and my address back here in the UK. A few weeks back I received a letter (in German) what I presume to be a demand for payment, I didn't want the hassle so I thought lets just pay the fine of 45 euro, which I did a fortnight or so later. A few days ago I received a 2nd letter which I presume is from a debt collection agency demanding a larger sum of 90 euro. Obviously during the time it has taken me to pay and the transaction to take place they have sent my details over to the debt collection arm. What should I do? the fine is paid but I imagine the debt collection agency will still peruse the whole amount and it's immaterial to them that the original fine has been paid in the meantime. I can pay the fine for the 2nd time, contact them to explain or simply ignore it. Can they peruse me here in the UK? I imagine if I ignore this 2nd letter I will end up with a criminal record in Germany and it might end up following me here in the UK.