If you ring Summerhill to ask where the bus is that for example doesn't turn up...and the physical bus is based at Harristown, you've a valid point alright..
When a driver logs on to the ticket machine on a 41X, they appear on the screen of the controller in Summerhill, so if you phone Summerhill regarding the 41X they'll have as much information, weather it be a Harristown or Summerhill bus.

In fact, if you phone Harristown, they may be able to tell you what bus was allocated, but their controllers will have drivers/buses from the 4/a, 13/a, 27b etc on their screen, and are not watching for 41X's unless its an emergency.

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