All interesting posts, thanks. I don't suppose a different splitting policy (say 7:3 is possible?).
It would be possible but your mainline trains would need to be loco hauled and you'd need passenger carrying
DVTs‡ at suitable points in the train to enable the split and push pull working.
The best exmple in recent years was the REP
TC‡ Class 33 operration from Bournemouth to Weymouth. Where the 4REP pushed 2 4TCs to Bournmouth a 33 came on the front and depending on time of day pulled 1 or 2 of the TCs to Weymouth. Coming back it pushed the TCs from Weymouth and coupled onto the REP in Bournemeouth station.
The trouble is we've rationalised most station layouts so that there are now no loco spurs to enable engine changes eg from electric to diesel.