First off, you have to know 1 MP location on your route, if it's a real world route. If it's fictional then the start can be anywhere you desire.
MP locations can be gotten from track charts or from timetables, though timetables will only give you a general sense of where they would go.
Track charts can be bought from;
https://railfandepot.com/product-catego ... ck-charts/They are typically $10 for the .pdf digital download and the upper price range is if you prefer a printed copy.
I place my MP's in the following manner.
Determine where a first one will be placed.
Create a new scenario and name it something like 'Mileposts' at that location and place your first MP down.
Place a portal somewhere down the line, I usually go 10 to 15 miles down the line.
Click on the Scenario Tools icon and place an engine with the front coupler even with the first MP you placed. Click on the Driver icon and place one on the engine. Double click on the driver icon that is on the engine and after the dialog box is up hit the #9 key for the schematic track view and navigate to and zoom in on the portal you placed.
Click on the Set Destination icon and then click on the portal in the #9 view. The name of the portal should now be displayed in the Set Destination field.
Add 1 or 2 minutes to the Start Time so the engine doesn't take off as soon as you start the scenario.
Start the scenario and click on the engine. You'll see the portal name as your destination in the HUD along with a distance displayed down to the hundredths of a mile. For example, if it displays 14.67 miles you know that the next MP should be placed at 13.67. Start moving and when you get to 13.67 hit esc and go to the editor to place your 2nd MP. F2 to save the changes and hit run and your off to the next one at 12.67
FYI, the game doesn't like going back and forth between driving and the editor a bunch of times so you may crash with an Out Of Memory error somewhere along the way. Just restart the scenario and run up to where you placed the last MP and continue placing the MP asset..
Once you reach the portal, or I should say the MP just before the portal, which in this example would be at .67 you can delete this scenario and create a new on at this last MP location so you don't have to spend time driving up to the last MP. Place another portal 10 or 15 miles further on and repeat the process. Of course, it will no longer be the same calculation to the next MP, it may be something like 12.58 miles to the next portal, so you would now stop and place a MP whenever you get to XX.58
Download the PRB from page 51 of the Powder River Basin thread.