Debug Code With Reasoning

BNSF Railway's dispatch team uses an internal dashboard to track railcars on a given train. A candidate for Frontend Engineer was shown this component and asked to debug it while narrating their thought process out loud.

import { useEffect, useState } from "react";

interface Railcar {
  id: string;
  status: string;
  lastStation: string;
}

function ShipmentTracker({ trainId }: { trainId: string }) {
  const [railcars, setRailcars] = useState<Railcar[]>([]);
  const [delayedCount, setDelayedCount] = useState(0);

  useEffect(() => {
    fetchRailcars(trainId).then((data) => {
      setRailcars(data);
      const delayed = data.filter((car) => car.status === "DELAYED");
      setDelayedCount(delayed.length);
    });
  });

  function markInspected(id: string) {
    const car = railcars.find((c) => c.id === id);
    car.status = "INSPECTED";
    setRailcars(railcars);
  }

  return (
    <ul>
      {railcars.map((car, index) => (
        <li key={index}>
          {car.id} - {car.status}
          <button onClick={() => markInspected(car.id)}>Inspect</button>
        </li>
      ))}
    </ul>
  );
}

export default ShipmentTracker;

Debug this component out loud: what would you expect to actually happen when it renders and when a dispatcher clicks "Inspect," and what would you change and why?