Complex sentence: Peppercorn finished several projects which were started by Thompson, but most popular were his LNER Peppercorn Class A1 and the LNER Peppercorn Class A2.
Split into simpler sentences: Peppercorn finished several projects which were started by Thompson. But most popular were his LNER Peppercorn Class A1 and the LNER Peppercorn Class A2.