I imagine that at least two things would make it difficuilt to put a Bluebird trailer amongst Hitachis is the lack of control connections through the Bluebird
The Bluebird trailers do have a control cables since they could operate between Bluebird motors. Whether or not they are compatible with Hitachi control systems is another matter. If not you could run Hitachi-compatible bus lines through the trailer. Not cheap, but doable.
and possibly that the Bluebird would not physically couple to the Hitachi.
This is the "possibly two" I mentioned earlier. Hitachis have link and pin couplers between cars. It may
be possible to fit an auto coupler into the pocket but I'm not 100% sure. Open to advice on this one.
I would also have doubts about the strength of a Bluebird when in the middle of a consist.
Better not tell FreightLink who use them as crew cars at the head of 3000t trains!
Come on, bigger reason than that. Monday was close when he mentioned electric control equipment.