On Montana Rail Link I have seen block coal and oil trains operated by BNSF with 2 BNSF locos leading, 4 MRL helpers (manned) mid train, plus 1 BNSF DPU trailing at the end of the train. How does this mix of manned helpers and DPU work? These trains operate in mountainous country where radio communication must be poor. The trains I describe can be seen on You Tube.
Spent the last couple of weekends in the Emerson, GA area at baseball tournaments and saw a few trains on the W&A Sub. Everything was 2 x 0 x 0 except for in that look like it had a couple of GPs DiT behind the AC’s
The only thing I know, is that you can’t just place it randomly where you want, from what a guy working with the locomotives told me.
If you don’t respect the rules, it might end up getting messy, so it’s better to ask a professional for it. It reminds of my last trip by train, the last year, when me and a few friends of mine started a journey across the country with https://www.dbfahrplan.com/de/, and looking at this image, those memories come to my mind, what a beautiful adventure it was, wanna repeat it again!
(1) spread the drawbar forces as evenly as you can.
(2) then be concerned about the delivery of the air.
I worked with an old head Trainmaster/RFE/Asst. Supt. who could accurately figure out where the coupler/drawbar pull apart failure was simply by looking at the trains makeup printout at departure.
2/3rds of the way back is by tonnage, not by car count or length.
Here is a basic primer from Indian Railways with a couple of good illustrations of how DPUs affect power at the drawbar, and force against rail on a curve.
https://irimee.indianrailways.gov.in/instt/uploads/files/1434535190830-DPCS.pdf