Complex sentence: Track ballast typically sits on a layer of sub-ballast; the latter is typically made of small crushed stones, giving a solid support for the top ballast, and seals out water from the underlying ground.
Split into simpler sentences: Track ballast typically sits on a layer of sub-ballast; the latter is typically made of small crushed stones. It gives a solid support for the top ballast, and seals out water from the underlying ground.