Something I noticed since tsw 5 was launched is that now you can't modify any of the new multiple units trains in formation designer, meaning that instead of appearing as all cars separated they only appear as one train that can't be modify (so far problem with classes 170, 350, 390, 710 and db br 411 and 423). I don't understand why dovetail stopped putting efforts in it because all the old EMUs can be modify (like class 377, 395 or all the ICE). I really hope they will update it cause it's pretty much a like walking backward in the evolution of the game.
I can only tell for the 411 and 423. Modifying the formation would break the functionality of the unit. These units contain of spread out aggregates all over the different cars. But all of them are needed to run the train. Removing one car, or adding one will break it.
I get that there are some things that you can't do like trying to remove the car with a pantograph will of course forbid the train to move but still trains similar to that like the ice 3 can already be modify so I doubt it would cost them anything to just put it on the formation designer and worst case scenario the formation doesnt work but at least you would have tried.
I have uploaded some of the missing formations to creators club now: Class 350 - DMOS1, PTOSLW, TOCL, DMOS2 Class 390 - A (DMRFO), B (MFO), C (PTFO), D (MFO), E (MSO), F (TSO), G (MSO), H (MSO), I (PTSRMB), J (MSO), K (DMSO) Class 710 - DMS A, DMS B, MS, PMS BR 411 - 411.0, 411.1, 411.2, 411.8, 411.7, 411.6, 411.5 BR 423 - Front Cab, Pantograph, Trailer, Rear Cab Bigger list can be found here
