If I have a railpass for Germany and France and only want to see Luxembourg City without buying the pass for Belgium Holland and Luxembourg. How do I pay for it? I will be travelling from Trier to Luxembourg City and then onto Paris. My Eurorail pass will only take me to the border. Do I buy my ticket in Germany for Luxembourg or when it crosses the border and the same for when I leave for Paris?
Yes, buy your ticket in Trier for your ride from the German border to Luxembourg City and in Luxembourg City for your ride to the French border.
FYI, the cost of a standard ticket from Trier to Luxembourg is €13.70 and from Igel (the border station) it's €12.10. You could buy a standard ticket all the way and save a pass day.
For the ten-minute ride from Luxembourg City to Bettembourg (the border station), the fare is €4.00.
We stayed in Trier and took a day trip to Luxembourg City to visit the American Cemetary. The fare (point-to-point) was inexpensive.