This link [1] says the median US train is 5,400 feet, and this link [2] quotes an intermodal railcar for 53-foot containers at 67 feet, 9 inches; although 40 foot containers are the most common size; let's use 60 feet as a round number car length; that would put the median train at 90 cars; the 90%ile train is 9,800 feet or 163 cars by my length estimate, and 99%ile was 14,000 feet or 233 cars.

Probably depends on the specific route, but 200 cars doesn't seem unrealistic.

[1] https://www.aar.org/article/freight-train-length/ [2] https://www.mrc-rail.com/railcar-results/intermodal/