A dolly is an auxiliary axle equipped with a fifth wheel that allows coupling a conventional semi-trailer as a trailer via drawbar. It is the key component in megatrailer (25.25 m) and ecocombi road train configurations, combining rigid truck + dolly + semi-trailer.
A single-axle dolly is lighter and cheaper, suitable for moderately loaded semi-trailers in light EMS configurations. The twin-axle distributes the load better and allows a higher combination GVW, essential to approach the 60 t of Nordic mega-trains. Check the fifth-wheel condition (play, slide plate, king-pin lock), the drawbar, the eye coupling and the BPW/SAF axles. Coatrucks states the number of axles, fifth-wheel type and towing capacity on every dolly listing.
The dolly is registered as a standalone trailer and needs its own type approval and MOT. To run in megatrailer (25.25 m) or ecocombi configuration in Spain you need special DGT authorisation and approved routes, since the ordinary limit is 18.75 m. For intra-EU import it's worth confirming the dolly has a COC or documentation that allows its homologation. Coatrucks handles the import and Spanish registration of the dolly and advises on the requirements of the road-train configuration you want to build.
Dollys for ecocombi and road trains.