Final answer:
The narrator cares about what side of the train he sits on to avoid being weakened by the water.
Step-by-step explanation:
The narrator in the passage cares about what side of the train he sat on because of Mr. Sieppe's warning about the water. Mr. Sieppe mentioned that if the water is too warm, it weakens you. When the local passenger train passes by, Marcus puts a nickel and two crossed pins on the rail, and they get flattened. The party then competes for the possession of these flattened pins, and it becomes a matter of tremendous moment to Mr. Sieppe. The narrator wants to sit on the side of the train away from the drain to avoid being weakened by the water.