I also recommend using the  Ronkonkoma - New York Penn service, but the way I'd recommend doing it is a bit different.
1. Start up the service and get the train moving and up to 80mph
2. Set the throttle to Min Power
3. Pause and make a save.
4. Walk away and do whatever you want for ~11 minutes
5. Come back and just before the speed limit drops to 35, quit to the main menu
6. Load up Last Played and walk away for 11 minutes again.
7. Repeat until achievement pops.
Advantage to this is that you don't have to go through the hassle of train startup every time, or have wasted time with the train covering less ground while it accelerates up to 80mph. Disadvantages to this are that you have to check on it more often (11 min instead of 15-20), and if you're not on an XB1X the load times can be brutal, but on my X it was about 15 seconds back to the main menu and about 30 seconds to reload from the main menu back into the train running.
The train will also slow down a little dropping to 68-ish but going back up to ~72 throughout the run based on the incline grade at various points, but if you give it more than Min Power it will go over the speed limit and then you're not getting credit at all, so losing a little speed is the compromise for not having to babysit it and being able to do other things while it just runs on its own.
Using this method I went from 37% to 100% in about 8 hours.