"It's easy to pretend innocence when what was done, happens in another place." Anyone can have a clear conscious when they aren't faced with their actions every day. The train provides an escape as much as it is a prison.

He has been watching the screen this entire time but hasn't really seen anything that has been playing out. But he knows these are the two she is invested in so he actually simply stays quiet. This could almost be seen as kind when it comes to him, considering he should want to distract her from finding out their outcome.