response to its Remedial Plan Notice, First Great Western undertook a more thorough refurbishment of the Thames Turbo fleet than originally planned: the trains were to be fitted with improved lighting, carpets, toilets, and a revised seating layout. This refurbishment started in September 2016. Class 166 Networker Turbo The "Networker Turbo" is a three-coach DMU, similar to the Class 165 units but with an internal layout more suitable for longer-distance services. They are now mostly based at St Philip's Marsh depot in Bristol, where they currently work on most of the lines in the area including the Wessex Main Line, Severn Beach line, Heart of Wessex Line, Golden Valley line and Bristol to Taunton line. Class 387/1 Electrostar The "Electrostar" is a four-coach EMU built by Bombardier, with a 2+2 seating layout, tables, power sockets and free Wi-Fi. It can be operated in four, eight- and twelve-coach formations. The class began to enter service in September 2016 on weekday peak services between London Paddington and Hayes & Harlington, using the overhead electrical equipment used by Heathrow Express. Services using the class were extended to Maidenhead in May 2017 and later to Didcot Parkway, and from Reading to Newbury. Bombardier Transportation at Ilford Depot had modified twelve of these trains by December 2020, installing new first-class seating, Wi-Fi, luggage racks and on-board entertainment, to be used on Heathrow Express services. Rebranded as "Heathrow Express", and refurbished with Heathrow Express moquette, they replaced the existing , entering service on 29 December 2020. West of England services Class 43 + Mark 3 HST / Class 255 Castle Great Western Railway will be retaining 24 power cars and 48 carriages from its former High Speed Train fleet to form 12 'Castle' 2+4 sets. They are branded as Class 255 sets and will be for use on multiple services between Cardiff, Exeter and Penzance. All power cars being retained will have new nameplates, named after castles from across the area that GWR serve. The sets are progressively being fitted with automatic doors and controlled emission toilets, to allow their operation beyond 2020, at Doncaster Works. Due to a delay in refurbishing the Castle sets, slam door 2+4 sets known as 'Classic' sets were used until the end of 2019. Until 2017, GWR operated the vast majority of its long-distance services with a fleet of 58 InterCity 125 High Speed Train sets, each consisting of eight