There are various reasons for operating a CBTC system with an external electronic interlocking (CBI). For this purpose, it is necessary for the electronic interlocking to provide a suitable interface that will enable integrated and convenient operation. Mixed operation of vehicles equipped with CBTC and conventional vehicles, where complex operator actions and processes are undesirable, is also often required. An interface and the necessary functions in the electronic interlocking that meet these requirements are described in this article.