Kit for connecting a semi-trailer of an articulated unit, where said semi-trailer comprises a semi-trailer chassis and a coupling pin for a fifth wheel of a road tractor in the front part of said semi-trailer chassis, where said road tractor comprises a truck chassis and a fifth wheel assembled on said truck chassis, said kit comprising: [a] a coupling pin for a fifth wheel of a road tractor, suitable for being attached in the rear part of said semi-trailer chassis, [b] a plurality of first anchoring elements suitable for being joined to said semi-trailer chassis, [c] two first attachment elements suitable for being joined to said semi-trailer chassis, [d] a bumper with a collapsible mechanism, suitable for being joined to the rear part of said semi-trailer chassis, [e] a plurality of second anchoring elements suitable for being joined to said truck chassis, [f] a second attachment element suitable for being joined to said truck chassis, [g] two box chassis, [h] a plurality of lifting means suitable for being arranged vertically under said truck chassis, said lifting means being able to raise said box chassis, [i] each of the two box chassis in turn, comprises: a plurality of support elements, third anchoring elements suitable for anchoring in said first anchoring elements, fourth anchoring elements suitable for anchoring in said second anchoring elements, a horizontal cylinder with a rod at the end of which there is a third attachment element suitable for being attached indistinctively in said first attachment element or in said second attachment element, and a hollow space suitable for housing in its inside said fifth wheel. [a] a coupling pin for a fifth wheel of a road tractor, suitable for being attached in the rear part of said semi-trailer chassis, [b] a plurality of first anchoring elements suitable for being joined to said semi-trailer chassis, [c] two first attachment elements suitable for being joined to said semi-trailer chassis, [d] a bumper with a collapsible mechanism, suitable for being joined to the rear part of said semi-trailer chassis, [e] a plurality of second anchoring elements suitable for being joined to said truck chassis, [f] a second attachment element suitable for being joined to said truck chassis, [g] two box chassis, [h] a plurality of lifting means suitable for being arranged vertically under said truck chassis, said lifting means being able to raise said box chassis, [i] each of the two box chassis in turn, comprises: a plurality of support elements, third anchoring elements suitable for anchoring in said first anchoring elements, fourth anchoring elements suitable for anchoring in said second anchoring elements, a horizontal cylinder with a rod at the end of which there is a third attachment element suitable for being attached indistinctively in said first attachment element or in said second attachment element, and a hollow space suitable for housing in its inside said fifth wheel. a plurality of support elements, third anchoring elements suitable for anchoring in said first anchoring elements, fourth anchoring elements suitable for anchoring in said second anchoring elements, a horizontal cylinder with a rod at the end of which there is a third attachment element suitable for being attached indistinctively in said first attachment element or in said second attachment element, and a hollow space suitable for housing in its inside said fifth wheel.