Complex sentence: Services to Walton-on-the-Naze are normally Class 321, Class 360 or occasionally Class 315 EMUs, while trains to Clacton are normally Class 360 EMUs and Class 321s.
Split into simpler sentences: Services to Walton-on-the-Naze are normally Class 321 and Class 360 EMUs. Trains to Clacton are normally Class 360 EMUs and Class 321s.