Do you realise how long it takes a driver to "learn" a route? And do you realise that to "learn" another route means he is not going to be available for his usual route whilst under instruction? Signal boxes are the same. They may all look the same, but I wouldn't be allowed to walk into another Box and work it without training. In a simple Gr2 Box that might only take a couple of shifts, but on a big panel.................. I know that when I was off sick for over 12 months it caused all sorts of problems at my Box as I couldn't be replaced until I had finally left.