Although Schwertransport(e) may refer to the vehicle(s) or trailer(s) (more often Schwertransporter), is it not the case that here it refers to the actual heavy/oversize/overweight load itself? In that case, it would be the load that is self-supporting.
See Collins:
Schwertransport m (Ladung) heavy load (=Transporter) → Schwertransporter m heavy truck or lorry (Brit), heavy goods or heavy weight (US) vehicle form, HGV (Brit)
http://dictionary.reverso.net/german-english/SchwertransportE.g.:
Today we are primarily a heavy haul carrier. We transport oversize, overweight and difficult loads, specializing in self-supporting loads.
http://www.perkinstrucking.net/and offers the advantages of transporting large self-supporting loads such as steel tanks, large generators and more
http://www.khl.com/iphone/detail/item68179/Examples of self-supporting loads are heavy machinery having a steel base with mounting feet, construction machinery such as rubber-tired front-end loaders, scrapers, and rough-terrain cranes.
http://pittstrailers.com/app/inventoryapp/warranty__manual_i...http://de.wikipedia.org/wiki/Schwertransporthttp://en.wikipedia.org/wiki/Oversize_load