Operates as one large train. They get to a certain point and they are split. First half goes
where it's supposed to and the second half originates at the split off point and goes where it is supposed to. Most common is combining empty grain trains, but I've seen empty grain combined with an empty oil, two empty oils combined, empty grain combined with an empty coal.
Current issue of Railfan has a photo of two combined loaded coal trains operating in Texas (I think.) with a photo. The empty trains that operate over MRL often are split at Laurel.
I haven't heard where the hiline trains get split most often. Highest axle count I've heard was in the 1200's.