Because I have tortuous gradients on my line (peaking at about 1:12) I always wanted to find a system of holding back the coaches/cars on the downslope.
But I did not want any more drag going up the hills.
Back in 2006 when I first built the railroad I saw an inertia car on a railway that I visited and it operated very well indeed but great as it was on a level layout, on mine it would just cause far more problems climbing the hills.
My layout is both DCC and DC (not at the same time!) and I run R/C battery and live steam.
As I was multi-heading for long trains ( when using DC) I found that by finding the slowest loco and putting that towards or at the rear of the cars, it worked really well on holding back the train on the downward slope and then, even though it was the slowest, it still added some oomph on climbing as the forward loco(s) would slow up a bit taking up the strain of the train.. (with DCC I could 'make' one loco slightly slower by adjusting the CV).
But I still wanted single steam-loco running with a decent number of cars behind, so I built motorised freight or passenger cars and placed into the string of cars to help the single loco.
The motorised cars were just a smidge slower than the loco so they also stopped the force on it when going down hill.
But when running live steam I have relied on R/C geared locos like Shays, Climaxes and a Heisler to haul the trains on their own on the challenging layout, ironically, as the prototype was designed to do.
Sometimes I double head the live steamers to haul longer trains.
I thought of making R/C battery motorised freight/passenger cars to enable a single rod-driven live steam loco to climb the gradients but controlling the balance for the battery 'helper/retarder' car(s) and the live steam loco might need some very quick thinking and concentrated handling of the R/C transmitter(s).
Maybe that might be the way to go...