NMBS-etiquette

I can ignore a lot of shit in the Belgian train circuit. Like I have given up on the ‘silent coupée’, I have accepted that trains can be loaded and people need to stand for 30+ mins because they refuse to open up first class….
But what I can’t stand and also really don’t understand is people taking off their shoes and resting their dirty dogs of feet on the opposite seat. I am the only person who is truly grossed out by this? Should I lower my expectations when it comes to how people behave in public? Tell me, what is the correct way to tell a fellow passanger to behave themselves? Is there one, or is there only the Flemish way of complaing behind each others’ backs?