The basic difference
Tractive effort is pulling force at the rail. Horsepower describes the rate at which work can be done. A locomotive must turn engine power into usable force without slipping its wheels.
Starting a heavy train
At very low speed, adhesion and available tractive effort are critical. Modern wheel-slip control helps keep traction near the useful limit.
Speed matters
As speed rises, horsepower becomes increasingly important because maintaining pulling force at higher speed requires more power.
Why tonnage ratings vary
Grades, curves, weather, train resistance, locomotive type and operating requirements all affect how much tonnage can be handled. There is no universal one-locomotive-equals-one-tonnage rule.
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