A tightly packed railway timetable can move many trains when everything runs exactly on schedule. A small delay, however, may force later trains to wait for the same track and spread disruption through the service. Extra time between planned movements can absorb some of that shock.
A compact train sequence is first disrupted by an imposed delay that spreads to following services. A revised timetable adds margin at useful points so the same disturbance produces less knock-on delay. The final act reveals the price of that protection: more space between train paths means fewer trains can be scheduled through the bottleneck in a fixed period. Robustness is therefore not free capacity. It is a deliberate trade between dense use of infrastructure on good days and reliable operation on bad ones. The illustrative sequence makes that trade visible without claiming to identify a universally correct amount or placement of slack.
A port has a limited length of quay, and a ship must occupy one continuous stretch of it for the duration of service. Assigning an arriving vessel to the first apparently open place can block another vessel that needs a different length or arrival window.
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An electric train can return energy to the traction system while braking. Another train can use some of it if that train is accelerating at the same time and the electrical connection can carry the power. Without such a match, the opportunity may be lost even though the braking train still produces energy.
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Quay cranes are shared resources. Splitting them evenly between two ships may seem fair, yet keeping both ships in service longer can delay a third vessel that needs a berth. The time at which a ship leaves can matter more than how busy every crane appears at each instant.
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