While I don't know the exact answer to your question, I can tell you that on the Thalys train we took from Paris to Dortmund on a Sunday morning, the conductor passed through our car after every station we stopped. Will conductors keep an eye out for those whose ticket origin isn't the previous stop? I don't know. Will it matter to conductors? I don't know that too. I can just say that a conductor checked for new passengers after each stop.