This expression is similar in meaning to “packed like sardines”. Sushi, the combination of raw fish with vinegared rice, is popular in Japan and increasingly so in the United States and Europe. Take-out sushi is often bought in little boxes called sushi-ori, in which the sushi pieces are packed tightly.
A: Atarshii shigoto, tanoshii kedo, tsuukin ga taihen de ne. Maiasa sushizume no densha de ichijikan na n da yo.
B: Taihen da ne.
A: My new job is a lot of fun, but commuting is terrible. Every morning I spend an hour in a jam-packed train!
B: That’s awful!