The coupling constraints on the tub-boats are incorrect: the next constraints are set to allow them to couple to other tub boats, but the previous constraints are not, with the result that they cannot form a train. The following needs to replace the coupling constraint code in all of the tub boats' .dat files:
Constraint[Prev][0]=BoatHorsesSingle
Constraint[Prev][1]=TubBoatBulk
Constraint[Prev][2]=TubBoatLong
Constraint[Prev][3]=TubBoatPiece
Constraint[Prev][4]=TubBoatCool
Constraint[Next][0]=TubBoatBulk
Constraint[Next][1]=TubBoatLong
Constraint[Next][2]=TubBoatPiece
Constraint[Next][3]=TubBoatCool
Constraint[Next][4]=none
Edit: There is also a typing error in the constraints for the steam narrowboat tug, where the name is spelt "SteamNarowboatTug" instead of "SteamNarrowboatTug" in the constraints but not the object name, meaning that nothing can be pulled by the steam narrowboat tug.