This might be a bit of a strange suggestion since I know that Fornation Designer was supposed to have normal limits for what you can and can't do. But I genuinely feel like it'd be a pretty neat feature if we could change the coupling itself on each end of the BR 401 Trainset in order for normal locomotives to be able to pull the cars. Same goes for the LGV Duplex Coaches which CAN currently be be pulled by normal locomotives AND attached to them in formation designer, however it tends to glitch out and merge every single one into one slot.
