Complex sentence: Trailers were to be retired first, in the early part of the 1960's, as a matter of practicality since by this time the use of trailer cars in the New York City Subway was being phased out.
Split into simpler sentences: Trailers were to be retired first, in the early part of the 1960's. This was a matter of practicality since all trailer cars in the New York City Subway was being phased out.