The only free seats were on either side of that person.
The only seats that were empty were on either side of that person.
「(地下鉄の席で)あの人両側の席だけ開いていた。」＝ The only free seats were on either side of that person. / The only seats that were empty were on either side of that person.
ボキャブラリー
only ＝ だけ
free seats/seats that were empty ＝ 空いていた席
either side ＝ 両側
that person ＝ あの人
イギリス 
