Therefore he did not see the young man who stepped forward to shake her hand and whisper in her ear.

This young man was Sam Bartlett, and, as a "city dude," Issy loathed and hated him.
No, Issy did not see the hurried and brief meeting between Bartlett and Gertie Higgins, but he had seen enough to cause forgetfulness of mundane things.

For an instant he stared after the vanished vision.

Then he stepped blindly forward, tripped over something--"his off hind leg," so Captain Sol afterwards vowed--and fell sprawling, the express package beneath him.
The crash of glass reached the ears of the depot master.

He broke away from the conductor and ran toward his prostrate "assistant." Pushing aside the delighted and uproarious bystanders, he forcibly helped the young man to rise.
"What in time ?" he demanded.
Issy agonizingly held the package to his ear and shook it.
"I--I'm afraid somethin's cracked," he faltered.
The crowd set up a whoop.