Custom assignment of motor cars to a train

It would be useful to be able to properly determine which cars in a train formation are powered and which are not. Currently, unless the powered cars are at the end of train formations it is quite difficult to get the motor sound to play on the correct cars in some formations. For example, British class 321 has one motor car as the 2nd or 3rd car in the 4 car formation but it is difficult to get the motor sound to play out of the designated middle motor car. The .XML file format does not really fix this either. Is there another way this could be added? This might be a stretch but what is the feasibility of creating an updated train.dat file (maybe named train2.dat) to house expanded features?