And where did you search for that? Rail Europe I'm guessing. They only show big buck train rides they can make money on. Off the top of my head without a search, I'd say Boppard to Koblenz to Trier to Luxembourg. It is all regional trains that Rail Europe can't make money on. Try www.bahn.com to see real schedules.
Edit, actually it is a direct train from Koblenz. Boppard to Luxembourg takes 2 hours and 50 minutes with hourly connections through Koblenz for 44.90 EUR.
How many traveling? There is a sneaky, but legit way to do it for less. Buy a Rhineland-Pfalz ticket for 24 EUR (and 5 EUR for each additional passenger) and leave after 9 am (weekdays, anytime weekends) then then buy separate tickets from Igel to Luxembourg for 16.20 EUR per person. You don't have to get off the train. The R-P ticket is valid to the German border, then the Igel-Luxembourg ticket kicks in. If traveling by yourself, the total cost would be 40.20 EUR, saving you a whole 4.70 EUR. If more than one traveling, savings go up.