Strain relief is any feature that stops bending, pulling or vibration from reaching the crimp inside a connector. Without it, the wire flexes at the point where it enters the terminal, strands fatigue and the circuit fails, often with the insulation looking intact. Common forms are a rear shroud, a tie-off point, and a moulded boot.
Overmoulding is one way of making that boot. Plastic or rubber is moulded directly around the connector rear and the cable behind it, which bonds the two, seals the entry against moisture and gives a controlled bend radius. APAR's rail harness pages list overmoulds and strain relief among the features of complex harnesses. For routing and fixing practice, read the rolling stock harness routing guide, and see the locomotive harness page.