I know this locomotive can operate as a single unit it's not a locked into it's train set like some HST that share wheel sets,and somewhere i think i saw a class 43 being used as a frieght locomotive,so why not allow this locomotive to be used in other ways.
You would never have seen a 43 as a freight locomotive, they were built solely for high speed passenger work, the only non passenger 43's are the flying banana HST set used for track surveillance work.
In the real world sure,but when you can put another countries trains on routes they never would be found on,then this isn't to much of a stretch to be possible. Have you ever seen a... ICE3 in the US HST in canada GP38-2 on west summerset DB BR 143 on the island of wight 1972 stock Bakerloo train in germany So tell me again why a class 43 can't be used to do other things?(At least here in tsw2) Yes this is a sim game but off the rails put that idea on hold for awhile.
Because yes you can put a foreign loco on a different route but you can't change the style of train it can pull, when the HST's were built they were built as DMU's and not locomotive and coaching stock, the game will always keep the train formations realistic, even on a foreign line, that's just how the game is and probably won't be changing so your just have to accept that.