box, chest, case, bin, railway car
箱 centers on a container with defined boundaries: a box, chest, case, or bin for holding things. The railway car sense extends this to a large container on wheels.
箱 combines 竹 (bamboo) and 相, which likely contributes the sound. The character originally referred to a bamboo box or container.
The top 竹 suggests bamboo material, and the bottom 相 looks like a wooden box with a lid. Picture a bamboo box with a wooden frame: 箱 is a box.
For ソウ, imagine a box so big you can sew a whole chest inside it: sew -> ソウ, and the box holds everything.
box; case; chest; package; pack; crate
trash can; garbage can; rubbish bin; dustbin
miniature garden
ark (i.e. Noah's)
cased; boxed
jūbako; multi-tiered food box
birdhouse; nest box; bird box
Urashima's casket (in the story of Urashima Tarō); Pandora's box
shoe box; shoe shelf
packing into a box; boxing (up); thing packed in a box
vaulting horse; vaulting box; box horse
pencil case; pencil box; writing brush case
girl who has led a sheltered life; pet daughter; naive girl; ingénue
post office box
discarding; throwing away; getting rid of
police cell; lockup
box of 1000 ryō
large box; chest
incense box
toy box; toy chest