wagon, dray, canopied cart
輜 centers on a covered transport vehicle: a wagon or cart with a canopy, used for hauling goods.
輜 combines 車 (vehicle) with 甾, which likely contributes the sound and may relate to the idea of a covered or enclosed space. The exact historical development of 甾 in this context is uncertain.
A vehicle 車 with a canopy-like structure 甾 on top: picture a covered wagon used for transporting goods.
For シ, imagine a sheet covering the wagon: sheet -> シ, and the canopy keeps the cargo hidden.