Draco knew there was one debt paid during the war, but a second had yet to be fulfilled. Yes, Draco saved Potter's life at the manor, and in turn Potter saved Draco's in the Room of Requirement, but one incident had gone unmentioned: the bathroom duel. That meant Potter owed Draco anything he wanted… and the young Malfoy was going to do anything.
However, he couldn't just rush in and tell Potter that there was one debt left to be paid. He wouldn't be able to get the bloody Gryffindor to listen if that were the case. No, he needed to use a tactic to first get the idea in his head, get them to talk, and then Potter would do whatever he wanted.
It would be difficult seeing as both of them were in their Eighth year, and Draco was especially under the Ministry's suspicions. He couldn't exactly blame them for that though. If it wasn't for Potter speaking for him at the trial, he'd be locked up in Azkaban.
He decided to wait until the train to start his little plan…