The fastest way to complete this one is to choose Services mode, choose the 166 train, choose whatever weather you want, etc. Then, choose the "2P02 - Reading to London Paddington - 2:25" route.
With this route, you can crank your speed to max right at the beginning and since it is the first run of the day there are no other trains to cause red signals. You can cruise all the way in to Paddington at 90MPH and crash right in to the end of the tracks. Each run will get you roughly 2% of the achievement. So strap in, this will take a while. But you can idle it since there is literally no other interaction required once you get going.