Looking at this I'm tempted to make Teddy the rando sitting in the compartment with the Death Eaters. It'd be hilarious, especially considering Lucas's comment, as he's the only one who does know him. 

Just seems unlikely that Lucas wouldn't recognize him... and even more unlikely that he'd sit in an empty compartment by himself and then sit quiet for this long. No, it doesn't make sense by this point, but he'll definitely have to drop by and greet his [i]old friend[/i].