the present invention is directed to a modular storage system for storing items on a trailer . the modular storage system includes a plurality of containers connected to the railing of a trailer . the containers may be modular so as to be selectively arrangeable along the railing . the containers may be connected to the outside or inside of the railing . at least one of the containers may be positioned over a wheel of the trailer and include a wheel well for housing and protecting the wheel . the containers may include lockable lids for protecting the stored items . the containers may further include one or more attachments , such as a tool rack or t - mount .