Interoperability between stocks

As metman has touched upon, you have a couple of quite major technical differences......chief one being that the A & C stocks have Two air lines whereas D stock only has one!

A & C have Main line and Train line - D stock has Mainline with the Train line essentially replaced by a round train electrical circuit.

Braking systems? Without getting too technical about it, the brakes on A & C stocks are air controlled whereas D stocks are electrically controlled.

There's a whole host of other differences and technicalities which may or may not be insurmountable.

Getting past all those, you also need two compressors to make a train - these are located in trailer cars and are associated with a motor car as that's how they get their power.

If cost were no object, it might be possible, but otherwise you've got more chance of meeting President Lincoln that seeing such a train....