If you catch a Hammersmith westbound underground service from Paddington stn ( But get yourself Food/Drink first ) and alight at Royal Oak you can check and film the train in and out of Paddington from the westbound platform.
Then if you catch a westbound train to Hammersmith change to the District Line westbound to Ealing Broadway and you are back on the old G.W.R. the HST's go past the very fast also the New Coradia DMU's.
Then if you catch a 165xxx or 166xxx back to Paddington you have got Old Oak Common Depot on your left the EUROSTAR depot on your right just before you go under the North London Line also you have got Acton Stone Yard on your left and the class 59'S,66'S,and sometimes class 60's+ 67's