His imagination had not gone to any such refinement as the placing of a dummy copy where the original had been.
Quick as thought Jennie acted.

She slid open the door quietly and stepped out into the passage.

So intent were the two men on their work that neither saw her.

The tall man gave the box back to the conductor, then took the letter from between his knees, holding it in his right hand, when Jennie, as if swayed by the motion of the car, lurched against him, and, with a sleight of hand that would have made her reputation on a necromantic stage, she jerked the letter from the amazed and frightened man; at the same moment allowing the bogus document to drop on the floor of the car from her other hand.

The conductor had just emerged from Room A, holding his nose and looking comical enough as he stood there in that position, amazed at the sudden apparition of the lady.