GWR launched its electric services on September 2.
Class 387/1 units were initially used on Paddington to Hayes & Harlington services, with the service starting three days later.
GWR will operate two four-car sets in multiple to replace a two-car Class 165 and in the process create an extra 1,400 peak-time seats.
The units have free wi-fi, power sockets and larger luggage racks on all standard class accommodation,.
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