Regarded as essential for underground railways to have, emergency evacuation systems are now incorporated into all new railway designs which run in tunnels for long distances. The system usually requires either a floor level side walkway in the tunnel so that passengers can escape through side doors or a rapidly deployed end door to allow passengers to walk down to track level. The trouble with side walkways is that they require a wider tunnel than the train and this pushes up the cost of the tunnel construction. For a system very rarely, if ever used, this additional expense might be considered unnecessary by a prudent operator.