Complex sentence: All Tube stock was Third class only; all ex-MDR stock was Third class except those trailers identified as Composite; in the ex-Met ranges First class stock took the lowest numbers, then Composites, then Thirds.
Split into simpler sentences: All Tube stock was Third class only; all ex-MDR stock was Third class except those trailers identified as Composite. In the ex-Met ranges First class stock took the lowest numbers, then Composites, then Thirds.