Complex sentence: The British Rail Class 323 electric multiple units were built by Hunslet Transportation Projects between January 1992 and September 1995, although mock-ups and prototypes were built and tested in 1990 and 1991.
Split into simpler sentences: The British Rail Class 323 electric multiple units were built by Hunslet Transportation Projects. They were built from January 1992 through to September 1995, although mock-ups and prototypes were built and tested in 1990 and 1991.