Complex sentence: Cars BE 50 to BE 61 were converted from AE and ABE cars during 1981, as the start of the transition from wooden stock to all steel cars and altered rostering of carriages into small fixed sets.
Split into simpler sentences: Cars BE 50 to BE 61 were converted from AE and ABE cars during 1981. This was the start of the transition from wooden stock to all steel cars, and altered rostering of carriages into small fixed sets.