On the train, there has become an unspoken agreement to sit in every other seat (leaving one open seat).
Also, instead of standing in front of a seated person, stand near the door, etc. so that everyone can keep interpersonal space.
But, if it becomes crowded, just fill in the seats as usual.
「電車では席を一席分空けて座るのが暗黙の了解になりました。」は英語で「On the train, there has become an unspoken agreement to sit in every other seat (leaving one open seat).」という表現が良いと思います。
「また、座っている人の目の前に立たないで、ドアの近くに立つなど、対人距離をとるようにみんな工夫しています。」には「Also, instead of standing in front of a seated person, stand near the door, etc. so that everyone can keep interpersonal space.」という翻訳があります。
「でも、混んできたら、これまで通り、詰めて席に座ります。」は英語で「But, if it becomes crowded, just fill in the seats as usual.」という表現がよろしいです。