Once the railroad crossing barrier comes down, it takes quite a while until you can cross the tracks.
After the railroad crossing gate comes down, you have to wait a long time until you can go across to the other side.
ーOnce the railroad crossing barrier comes down, it takes quite a while until you can cross the tracks.
「いったん線路の遮断機が降りると、線路を渡れるまでにかなり時間がかかる。」
once で「いったん〜すれば」
railroad crossing barrier で「線路の遮断機」
cross the tracks で「線路を渡る」
ーAfter the railroad crossing gate comes down, you have to wait a long time until you can go across to the other side.
「線路の遮断機が降りたら、反対側に渡れるまで長い間待たないといけない。」
railroad crossing gate で「遮断機」とも言えます。
go across the other side で「反対側に渡る」
ご参考まで！
カナダ 
