It wasn’t his lawyer that speculated on the probability of a projectile, it was the assistant conductor that thought that she overheard a conversation between the engineer and conductor of the other train.
So she must have speculated and concluded that their train also was hit by some projectile.
Hearing about another train being hit by a projectile from communications with this engineer and the conductor of the “ other “ train could have been mixed up or the assistant conductor was confused by the conversation.