There are 2 separate issues here:
Proportional spacing: Unusual for typewriters, but the IBM Executive had it.
Line justification, i.e., even right margin: This had to be done by hand, as noted above. You typed it once, counted the spaces needed to right justify, marked where to insert the spaces, and typed it again. It would be a lot of work for a letter, but made mimeographed newsletters look "professional." I did a lot of justifying with a regular typewriter, but never tried it with an IBM Executive.

Thank you all for the very interesting comments. The discussion of the progression of typewriter technology and features has been helpful.

Going back to the purportedly 1946-vintage “letter”, despite its having been published by a historical organization many years ago, I have no confidence in the accuracy of a document appearing to be a communication from a railroad Superintendent’s office, that does not have any letterhead identifying the railroad, is printed in two different fonts (one of which is exactly the same as the page font of the magazine where it was published), and presents timeline conflicts with other properly documented sources including articles in company newsletters in the same period.

PC

Sounds like your suspicions are well founded, my BS detector would be on red alert.

Knowing what I do about typewriters of the period, I would be placing a straight-edge along the bottom of the letters to see if any are striking slightly above or below the line, and then looking for consistency within the document for that same letter. Also examine the impression from top of characters to bottom to see if any are striking evenly, and again consistent throughout. The ink fill of the characters will bear the faint semblance to the ribbon weave. Speaking of impressions, obviously the characters will be impacting the paper at different strengths, so the back of the paper is as interesting as the front. The paper would bear a watermark.

(I'm assuming you have it in your hands...)

It may be ever so slight, but no typewriter of the day would be perfect and these small identifying marks are often unnoticeable to the user. This certainly won't date the document but certainly will rule out modern era word-processing technology.

The original post mentioned perfect kerning. Kerning is more than simple proportional spacing, it involves adjusting character spacing so that some adjacent letters actually "overlap" in their spacing to create a pleasing visual effect.

For illustration, see the Wikipedia article.

A kerned document in the context of routine railroad operations would (as noted) be almost certainly a re-creation.

An early version of a form letter?
Below is a copy of a letter from 1897 that clearly has a specific address added to a standard text.

Notice how the signature block at the bottom is at an angle to the typed text. Looks like it was applied with a stamp.

My favorite fake document is one that was supposedly written by Franklin D. Roosevelt who passed away in 1945. When historian John Lyman send to the FBI,
they reported that it was written on a typewriter that came out in 1948.

Ted Miles, retired but still interested

I repair/restore typewriters professionally. Can you post a pic of the document or provide a sample of the typeface?