Modular first attachment system for attachment of a training rig to an upper edge of an ISO shipping container in its normal using mode, which first attachment system comprising: - a first attachment means which comprises: an inside corner bracket arrangeable inside a standard corner fitting of an ISO shipping container, an outside corner bracket arrangeable on an outside of a standard corner fitting of an ISO shipping container, first fastening means (16, 17), wherein the inside corner bracket and the outside corner bracket are connectable to each other by the first fastening means (16, 17), wherein the inside corner bracket is arranged to move in direction towards the outside corner bracket upon operating the first fastening means (16, 17), wherein the outside corner bracket and inside corner bracket are arranged to clamp a part of a corner fitting of an ISO shipping container, a first receiving part fixedly arranged to the outside corner bracket, wherein the first receiving part is arranged to receive an end of a beam, - a second attachment means which comprises: a first part and a second part, each arrangeable on an outside of a frame part of an ISO shipping container, a clamping plate which is arranged to be displaceably attached to the first part of the second attachment means, second fastening means (26, 27), wherein the clamping plate is arranged to move in direction towards the first part of the second attachment means upon operating the second fastening means (26, 27), wherein the clamping plate and the first part of the second attachment means are arranged to clamp a frame part of an ISO shipping container, a second receiving part fixedly arranged to the second attachment means, wherein the second receiving part is arranged to receive at least one end of a beam, - a longitudinal beam, wherein the beam is arranged to be connected to the first receiving part of the first attachment means and to the second receiving part of the second attachment means. - a first attachment means which comprises: an inside corner bracket arrangeable inside a standard corner fitting of an ISO shipping container, an outside corner bracket arrangeable on an outside of a standard corner fitting of an ISO shipping container, first fastening means (16, 17), wherein the inside corner bracket and the outside corner bracket are connectable to each other by the first fastening means (16, 17), wherein the inside corner bracket is arranged to move in direction towards the outside corner bracket upon operating the first fastening means (16, 17), wherein the outside corner bracket and inside corner bracket are arranged to clamp a part of a corner fitting of an ISO shipping container, a first receiving part fixedly arranged to the outside corner bracket, wherein the first receiving part is arranged to receive an end of a beam, an inside corner bracket arrangeable inside a standard corner fitting of an ISO shipping container, an outside corner bracket arrangeable on an outside of a standard corner fitting of an ISO shipping container, first fastening means (16, 17), wherein the inside corner bracket and the outside corner bracket are connectable to each other by the first fastening means (16, 17), wherein the inside corner bracket is arranged to move in direction towards the outside corner bracket upon operating the first fastening means (16, 17), wherein the outside corner bracket and inside corner bracket are arranged to clamp a part of a corner fitting of an ISO shipping container, a first receiving part fixedly arranged to the outside corner bracket, wherein the first receiving part is arranged to receive an end of a beam, - a second attachment means which comprises: a first part and a second part, each arrangeable on an outside of a frame part of an ISO shipping container, a clamping plate which is arranged to be displaceably attached to the first part of the second attachment means, second fastening means (26, 27), wherein the clamping plate is arranged to move in direction towards the first part of the second attachment means upon operating the second fastening means (26, 27), wherein the clamping plate and the first part of the second attachment means are arranged to clamp a frame part of an ISO shipping container, a second receiving part fixedly arranged to the second attachment means, wherein the second receiving part is arranged to receive at least one end of a beam, a first part and a second part, each arrangeable on an outside of a frame part of an ISO shipping container, a clamping plate which is arranged to be displaceably attached to the first part of the second attachment means, second fastening means (26, 27), wherein the clamping plate is arranged to move in direction towards the first part of the second attachment means upon operating the second fastening means (26, 27), wherein the clamping plate and the first part of the second attachment means are arranged to clamp a frame part of an ISO shipping container, a second receiving part fixedly arranged to the second attachment means, wherein the second receiving part is arranged to receive at least one end of a beam, - a longitudinal beam, wherein the beam is arranged to be connected to the first receiving part of the first attachment means and to the second receiving part of the second attachment means.