A few things:
There may have been a train on the main line.
Time is allowed in the schedule for the stop at Hansfield.
Some time may be allowed for delays, although this is usually put at the end of the trip.
They need to allow for the level crossing at Clonsilla to be closed, in case the train over shoots.
The reality is the train rarely needs to allow for more than one of the aboce, never mind all of them.
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