Complex sentence: Timber sleepers are designed to last 15 to 25 years, and they have to be replaced when they are near the end of their lifespan, or when they show signs of significant wear and tear.
Split into simpler sentences: Timber sleepers are designed to last 15 to 25 years. They have to be replaced when they are near the end of their lifespan, or when they show signs of significant wear and tear.