A rather unfair comment. The people who make these diagrams don't deliberately make them as tight as possible; indeed, after the software spits out a set of diagrams, planners would usually be expected to go through them to see if there's anything obviously wrong with them, or that could be made better by swapping work around between diagrams. Similarly, at most TOCs, diagrams are consulted with local union reps before coming into effect.
At my TOC the reps don't get to see the diagrams until they are produced, often in the same week that links have to be agreed. This allows little to no flexibility to sort out any problems with diagrams.
I do wish that management would have a better relationship with the unions, so when working on diagrams they can either be jointly discussed, or produced in time to sort any issues.
The consistent running down or trivialising of any railway employee who isn't frontline crew demonstrates incredible naivety. Just because you don't understand someone else's job doesn't make it worthless or an easy target for criticism. It takes a lot of people to run a railway, and that does include people who work in offices...
I think the view most traincrew have of diagramming is that too much faith is put into the system that compiles the diagram.
Very good points mentioned above, that walking time and allowances are agreed, but also there should also be an extra allowance - call it contingency planning, to ensure it does indeed allow for delays.
Stations and times changed, but this was my diagram yesterday.
Drive from station A to C, arrive 12:00.
Relieved by a driver (1)
Walking time (5)
Break (40)
Walking time (5)
Depart 12:52 passenger from station C to B and take over the train back to station A.
When you add up all the allowances on the diagram, that comes to 51 minutes.
There is 52 minutes between train arriving and departing. I have to be relieved by another driver, who does the disposing of the train because there is not enough time to do that in my allowances.
The train is the same train I work in to station C that I then travel on to station B and take forward.
So if I am delayed arriving into Station C, I have to take my break. They either hold the train for me to finish my break, or it goes without me and I have to travel on the next train to station B to pick it up.
Which ever way you look at it, this diagram was not produced with contingency, but it is 'legal' on all parameters.
For the sake of clarity, I would never not take my break, or take a shortened break just to keep a train running to time because if an incident was to arise it is me who has to explain to the ORR/Police why I didn't take my mandated rest. The company would not back me up, so I never take shortcuts.