This achievement is actually rather simple! From the main menu of the game, click the 4th image of a train to go to the Great Western Express
(hence the 'GWE' in the title of the achievement). Click on the timetable (or complete the tutorial first if you have not done so) and select the ‘Class 166 GWG’ train. Press the [E]-button to continue.
Having arrived at the timetables, I suggest selecting the ‘2P02
’-route from Reading to London Paddington
(departing around 2:25
am). This is the one I managed to get the achievement on, but I’m sure other routes work just as well. Press the [E]-button to start.
Having entered the cabin, start by unlocking the Master Key:
Then, set the Reverser
to be heading forward. This can be done by manually changing the position of the 'Reverser-stick' or by pressing [W] (when overshooting, press the [S]-button or use your mouse instead).
Then, set the throttle to 7. This can be done by manually shifting the 'Throttle / Break
-stick', but even easier by simply pressing (and holding down) the [A]-button on your keyboard.
When you let this set-up run for a few minutes, this achievement will be yours!
If the train doesn't start moving forward within a few seconds, make sure that your reverser is actually in the 'Forward'-position. Also, I have checked and you are also make it to 90 MPH (or 145 Km/h for people prefering the Metric system) with 'Disable Junction Derail' turned off (the standard setting) playing this route on the timetable. I still recommend to put this one on 'on' though!