Complex sentence: Once a new build of Class 455 EMUs were complete the 508s were slowly sent to their originally intended home on the Merseyrail network, to work alongside the already well-established fleet of Class 507 EMUs working there since 1978.
Split into simpler sentences: Once a new build of Class 455 EMUs were complete, the 508s were slowly sent to their originally intended home on the Merseyrail network. The 508 fleet then began to work alongside the already well-established fleet of Class 507 EMUs, which had been working on Merseyside since 1978.