One of the Edinburgh signallers told me that they'd had an incident some time ago when a class 66 on a route-learning trip had been abandoned in that siding at Waverley, blocking in the Electric loco for that night's Up Highland Sleeper. The signaller was waiting for a phone call from the Electric loco driver to say they were ready to shunt onto the sleeper - but the loco driver wasn't going to call when there was a "shed" between their engine and the siding starting signal! And at that time in the morning, it's tricky to find someone with traction or route knowledge to move the front loco. So ever since, they've been careful to use the TD to remind them what engines are in that siding.