Train Sim World 2020

Super easy solution.

From the main menu, go to Great Western Express, Then Services, then select Class 43 GWG, and select route "1C36 London Paddington to Cardiff Central." (The very last one).

To start the train, do the following:
1) Turn the master key from locked to unlocked
2) Turn the reverser to neutral (right below the master key)
3) Select the train brakes (left lever) and turn it to "running"
4) Wait a couple seconds then turn the train brakes to 3
5) Wait a couple seconds then turn the train brakes back to running
6) Turn the reverser to forward
7) Turn the throttle to max and let er rip. Don't worry about the 50 mph limit immediately.
8) - Wait 5 minutes as your train accelerates up to 125.

Nothing else is needed to do. My achievement did take a couple seconds after getting to 125 to unlock.