Bi-mode rail vehicles are likely to become significantly more important in the next few years than they were in the past, so it might well be worthwhile implementing code support for bi-mode vehicles at some stage, but, because this would be a lot of work and there are many other priorities, this is likely to be years away unless other developers join in to assist.
As to a top and tail locomotive hauled service, the reason that this cannot be done is that the coupling constraints of the locomotives at the front ensure that only locomotives with compatible multiple working types can connect together in a double-headed formation. There is currently no other way of doing this that does not also prevent top-and-tailing (which in any event would not allow the power of both locomotives to be used unless they are all part of a multiple working set such as an HST, in which this is possible, and there is no code support for having a locomotive only adding power when travelling in one direction in any event).
As to the costs of running obsolete vehicles, this is intended realistically to reflect the fact that it does indeed cost more to run obsolete vehicles. The actual costs for things, however, have yet to be balanced. Balancing will be a gargantuan exercise, and will only be able to commence after several further features (each one of which will involve an enormous amount of coding work) have been implemented.