A trailer is mobile due to the wheels it has, but cannot run alone as it has no engine, which means it needs a truck with an engine to be useful. To select the right trailer for the job depends on the type of load that needs to be carried. For example, if you must transport frozen goods, then a refrigerated trailer would be the one to pick.
IAM (Autodesk Inventor, filesize: 34.1 MB), STP (STEP, filesize: 34.1 MB)