1. Start any service with the 146.2 train.
2. Put the reverser to neutral and open the doors.
3. Wait for the Load Passengers circle to fill about 75% of the way and pause and Save
4. Let the ring fill the rest of the way and close the doors so that it says Objective Complete and quit to the main menu after.
5. Select Last Played from the main menu and keep repeating steps 4 and 5 until the achievement pops.
The tracker does not work for this achievement so you will have to keep track manually if you want to know how many you've done and how close you are to 146.