Remember that this was a retaining wall until very recently. Once the actual path of the track was certain, I used that to mark the location on the retaining wall. I then used a tunnel portal I had around that I knew cleared double-stacks to layout the actual opening, then took flexible retaining wall material and pinned it to that dimension. Another mark was made to show me where to cut in order to accommodate the flexible material. That was glued in place with Gorilla glue and pinned until it set. This is what's shown here. I later trimmed the excess, cut some mortar line grooves, and painted the new material.