Over the years I’ve come across some very long trains of grain and coal, but this recent video from Virtual Railfan is from one of their cameras in Missouri:

16K Roars Through La Plata, MO

People in the comments counted 240 or 269 cars. Let’s say it’s 250 cars. At 55’ length for each car, that is 13,750 feet plus (extra for the linkages and some slightly longer double height cars.)

That’s over 2.5 miles long.