Complex sentence: Unusually for a station on the Great Western Main Line, Burnham was built with platforms that serve only the relief lines, which makes it vulnerable to losing services when engineering work closes the relief lines and leaves trains only on the main lines.
Split into simpler sentences: Unusually for a station on the Great Western Main Line, Burnham was built with platforms only on the relief lines. This makes it vulnerable to losing services when engineering work closes the relief lines leaving trains only serving the main lines.