Sleeper Retaining Wall - PostCrete?

Not sure what I’m doing to be honest. Need advice if possible.

Building a retaining wall out of reclaimed sleepers. Six upright in holes 450mm by 350mm and 1m deep. Dug a 200mm footing and will go about 5 sleepers high, bolted to the uprights, just over 1m from the bottom of my footing.

For the uprights was going to add some gravel and maybe hardcore alongside them in their holes. Would postCrete be strong enough to set them in?