(31 May 2015, 10:35 am)Davie wrote I think it might take a while to get that many
Precisely this.
By the time we get to 5999 in the single deck series (580 vehicles away from 5419, which is presently the highest single deck fleet number), the vehicles at the start of the 5xxx single deck fleet number series will be long gone, so I see no reason why these numbers could not be re-used. They could even revert back to the 4xxx series fleet numbers - like Andreos1 said, single deckers such as Northern's Nationals were originally in this series and the oldest modern day Scania L94s are still in this fleet number series too.