Can you use concrete to fill holes in aging railroad ties in landscape

I have seen it done- never heard of problems afterward.

Probably not a good idea because they will eventually rot and then you'll have clumps of concrete to deal with. They also are loved by snakes and rodents for homes so I would dig up and replace with treated lumber, stone, or concrete blocks.

The cement will hold moisture and increase the rate that the ties are rotting. You would be better off to use a vinyl patching compound which does not hold moisture.

They will have to be replaced eventually anyway, so why not replace them now or put new ones in front of them if you don't want to take them out and you have the space. We did that in front of my tiered veggie garden where the only put one tie in and the soil was eroding out underneath it. There wasn't enough room to put one underneath it without serious digging out and we decided to just put two in front of the one to solve the problem. Had bush beans planted just on the other side of the ties and didn't want to disturb them by taking the one tie out to put in the second tie either.

covering railroad ties with cement