Busiest Subway Station

Problem

A transit network records daily train trips in a DataFrame `train_trips` (loaded from a CSV) with columns `(origin_station, destination_station, trip_total)`. The pair `(origin_station, destination_station)` is unique. Each row means `trip_total` trains ran that day from the origin to the destination.

A station's activity is the total number of trains that either started at it or ended at it. Return `station_id` for the station(s) with the highest activity. If several stations tie for the maximum, return every one of them. Row order does not matter.

Input data

Example rows — the live problem includes the full dataset.

Expected output

Your answer should return 1 row with the columns station_id.

Starter code (Pandas (Python))

import pandas as pd

def busiest_subway_station(train_trips) -> pd.DataFrame:
    # Your code here
    return train_trips

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