Depends on when it was built

Was the retaining wall built at the same time as the current turntable pit?  Perhaps the wall is older than the turntable pit.  Or, perhaps it is newer, and the economics at the time of construction were different.

In other words, I suggest that you do whatever you like, and justify it with an appropriate backstory!

It depends on the era and

Ditto to time,era and economics.

As well as availability of local materials.

Be fun

RJ

concrete years

If the retaining wall was built by the railroad after about 1890-1900, I would expect it to be reinforced concrete.  That range of years is when a lot of concrete construction showed up on American railroad structures.

If the retaining wall was built by locals for another purpose, then log or heavy timber cribbing could be used in lumber-rich areas, or cut stone if that's available locally.

Preliminary scenery

I want to build some preliminary scenery around the area which is why this has come up. It's a relatively low wall which would have been built for the turntable to go in so I'm thinking concrete but then, theres nothing to say that the TT wasn't updated over the years and the "old" wall was still functional so it remained the same. Any way, I think I'll just put up a "blank" and face it later when I decide what I want.  I have a piece of a wooden yardstick that looks about perfect for the blank wall.

Michael

Would it be plausible

to have an older stone or wood retaining wall being replaced with a newer concrete wall?  You could model it as an 'under construction' scene, very similar to the shoofly scene done in this months MRH magazine.  Just an idea.