Complex sentence: This third man would then take the bags with him, Slocum and Gilman would then toss their stuff off the train, and then rejoin the party in the dining car, without ever being missed.
Split into simpler sentences: This third man would then take the bags with him. While that was happening, Slocum and Gilman were to toss their stuff off the train, and then rejoin the party in the dining car, without ever being missed.