Complex sentence: The original (UTDC) Mark I cars are 12m (40ft) long while the second generation (Bombardier) Mark II cars are 18m (60ft) long each and come in articulated pairs.
Split into simpler sentences: The original (UTDC) Mark I cars are 12m (40ft) long. The second generation (Bombardier) Mark II cars are 18m (60ft) long each and come in articulated pairs.