Compounding the difficulties in providing a similar service are the Channel Tunnel safety rules, the major ones being the "half-train rule" and the "length rule". The "half-train rule" stipulated that passenger trains had to be able to split in the case of emergency.[3] Class 373 trains were designed as two half-sets, which when coupled form a complete train, enabling them to be split easily in the event of an emergency while in the tunnel, with the unaffected set able to be driven out. The half-train rule was finally abolished in May 2010.