Senior citizens

I want to go to Luxemburg with the family by train.
As a senior citizen I can get to Arlon and back for €6.00 but then there is a problem. If I book the last 20 minutes into Lux online it will cost another €22.00 return but there is a ‘tip’ on the website to say that booking in Arlon station itself is significantly cheaper. However that means that I have to get off the train, book the separate ticket and then wait an hour for the next train.
Has anyone done this and found a solution that doesn’t involve a choice between paying far more than you need to and wasting an hour in Arlon? (And I gather than the same applies if you want to cross into Aachen, Lille and so on.)