In yet another train first for us, we managed to find a GWR Class 378 train.  According to Wiki, The Class 387 Electrostar is an electric multiple unit (EMU) built by Bombardier Transportation.  

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It made us happy. And we watched some more at Ealing Broadway. 

The GWR Twitter people were very helpful in trying to in down where/when the 378 trains might be running. (Paddington to Maidenhead is your best bet)