Most countries in the European Union participate in an auto-gauge network. The United Kingdom is linked to this network via the Channel Tunnel. The Marmaray project connects Europe with eastern Turkey, Iran, and the Middle East via a rail tunnel under the Bosphorus. The 57-km Gotthard Base Tunnel improved north–south rail connections when it opened in 2016. Spain and Portugal are mostly broad gauge, though Spain has built some standard gauge lines that connect with the European high-speed passenger network. A variety of electrification and signaling systems is in use, though this is less of an issue for freight; however, overhead electrification prevents double-stack service on most lines. Buffer-and-screw couplings are generally used between freight vehicles, although there are plans to develop an automatic coupler compatible with the Russian SA3. "See" Railway coupling conversion.