One of the first things

One of the first things you will need to do is figure out physically how long your main line run is. Then, how many “scale” miles are you hoping to represent on your layout. If 12” = 1' on the real railroads, then doing the math approximately 60 real feet equal 5280 scale feet in HO scale, or 1 scale mile. So on my friends layout, I would only have had about 5 mile posts around the layout. Hardly enough for train crews to feel like they were going anywhere.

If you are modeling the Union Pacific from Chicago to St. Louis, then you might be trying to represent over 200 miles on a main line that is physically 200 feet long. Are you going to have a mile post every foot of track? Probably not. So maybe in that case you could selectively compress your scale miles. Find a number that's good for you. On my friend Lee Melton's HO Scale layout, which is where I added mile post sign, he has 300 feet of main line. We both felt that 25 real feet was a good round number to represent a scale mile. This gave me 14 scale miles of main line to plant posts. Choose a length that's right for you.

  We also both decided not

We also both decided not to start at 1. By starting at a higher number, it gave the trains coming from staging a sense of having come from somewhere. In this case I started at 36 and ended at 50. Both the first and last mile post are in staging.

To measure out the 25' spacing I used, I cut a piece of baling string 25' long. starting at the first post I planted, I stretched the string along the track until I reached the end, then planted the next mile post sign. Then starting over with the string from that point, and repeated this process around the layout.

A few small problems I ran into on the way.

A few areas of the layout are not very accessible. SO here I had to make my best guess where the string would start or end, to know where the next post would go.

Also, one of the mile posts fell on a portion of track that was hidden, so I am going to put a white label on the fascia to show where it would have been for timetable purposes.

These photos are of me stretching the string along the track, and of a finished mile post sign.

After they were all installed, I found a scale speed calculator to figure up how many feet an HO scale train traveled for a given scale speed. I translated the figures for our 25 foot long miles, and have that information in the layout room. So crews can now use a stop watch to check the scale speed of their trains, and when we add a dispatcher job to the operating sessions, he can give out track warrants from one mile post to another.

OTHER USE OF MILE POST MARKERS

Another option

Rather than spacing the mileposts evenly, you could use them to force the illusion of distance traveled.  For example, if one scene is set at the MP 8, and the next scene 20' away is set at MP 15, putting those mileposts up will reinforce the idea that you've traveled 7 miles in those 20'.

We use real mileposts

On the Sudbury Division we use real mileposts.  Since we are recreating a prototype route, and all the locations are real and on the timetable, we place reproductions of the real mileposts on our layout.  On hidden trackage, we also mark mileposts according to helix laps or whatever is appropriate so they can be referenced by train crews.  We regularly give clearances referring to mileposts for work limits, and crews are expected to know where those mileposts are.