What phenomenon caused broken couplings on very long trains when applying vacuum brakes?

Answer

The delay between the front piston responding immediately and the tail piston responding later.

When admitting air to apply the brakes, the pressure wave traveled slowly. The front pistons responded immediately, while the tail pistons responded much later, resulting in longitudinal forces that could break couplings.

What phenomenon caused broken couplings on very long trains when applying vacuum brakes?
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