It was a greater movie in 1940 than it is today, because back then, the railroad was much more a part of everyday life than it is now. People who were familiar with the real Broadway Limited would have better understood the urgency behind getting the train through on schedule, and the audience would likely have been on the edge of their seats as the train neared Harrisburg, where the train would be switching from steam to electric power for the run into Penn Station New York. Most people watching the film today wouldn't even know this was going to happen, whilst it would have been common knowledge back then.
Also, arriving in New York by train was a much bigger deal in those days, as the original Penn Station was far fancier than the one under Madison Square Garden is today.
Those exotic locales, which are mostly gone today, would have added to the intrigue in 1940. They are, sadly, lost on a modern audience.