[Closed] Cutting sleepers for raised beds

I've got some railway sleepers to make raised beds from, but need to be able to cut them square so that they sit right when butted at the corners.

I've got a chainsaw, but I'm not sure how easy it'll be to actually follow a line accurately enough to get a square cut?

Other tools available are first fix hand saw, circular saw (won't cut deep enough), sliding chop saw (again, not enough capacity) or jigsaw (!)

Any tips?