From #openttdcoop wiki
Variable Trainlength Handeling
It's sometimes usefull to have trains with different TLs on the same track. For example when you have cargo and passenger traffic on the same track or short primary cargo ones and longer secondary cargo ones. I thought of some constructions that make it easier to handle such traffic; one splits tracks with multiple TLs onto dedicated single-TL tracks, the other one is a prioririty construction which length depends on the length of the train waiting to join the prioritised line.