According to SAFe 6.0, System Teams are integral to the ART and are considered equal members. The recommended best practice is:

“System Team members should attend SAFe for Teams training together with all other teams on the train to ensure shared understanding of Agile, SAFe, and the train’s objectives and practices.”

This approach supports alignment, collaboration, and shared learning. Other answers do not ensure integration with the ART or are targeted at other roles or less comprehensive.