The sleeper hosts do not need to sign a route card, so the train manager will be the only safety-critical person with route knowledge which is essential in case of an incident (the locomotive may have become parted from the train, passengers might need to be detrained, the driver might be incapacitated; the TM needs to know exactly where the train is and what hazards are along the route/where the nearest station is). There could even be an incident whereby the driver fails to brake for a speed restriction or the brake pipe becomes isolated from the locomotive (as happened in August 2019 at Waverley); if the guard knows exactly where the train is he can put the brake in and potentially avert a very serious accident (the TM was praised for doing so in the RAIB report from August 2019). If the TM has no route knowledge, this effectively renders the service driver-only operated (DOO). Locomotive-hauled trains are not permitted to run DOO with passengers on board for these very reasons.