RWBY - RWBY characters_ Volume 6, chapter 1

And yet you still style your KYM account after Neptune.

I think there was a very small gap between the train and the tunnel wall. So all the damage of hitting the tunnel entrance wiped his aura, and then his arm got physically damaged from coming into contact with the train wall. He has a large bruise, which is usually impact damage but he probably had some internal bleeding in his arm and maybe his arm was broken till Jaune the White Mage came through.