Complex sentence: Of those, the older 53 are Alsthom TFS trams, whilst the newer 50, which started entering into service with the opening of the B line extension and the C line, are Alstom Citadis trams.
Split into simpler sentences: Of those, the older 53 are Alsthom TFS trams. The newer 50, which started entering into service with the opening of the B line extension and the C line, are Alstom Citadis trams.