While I can’t comment for sure on MR, in my business, reviews are done by the publications that the manufacturer supports by purchasing ads, so, I suspect the hobby industry is similar. If I was a model railroad manufacturer, I think I would make sure everything I sent to MR was painted BN green since Cody does a lot of reviews....
Years ago MR said they did both and also that they were always on the lookout for receiving a hand selected sample from a manufacturer that was not representative of what a real consumer would get. Getting what a real consumer would get was probably easier to do back when most production of rolling stock and locomotives was intended to be regular inventory on a dealer's shelves.
I was told that years ago for some very expensive or rare items such as some brass MR was asked to return it after testing was over but mostly they got to keep it, and I know that in some cases members of the staff could put it dibs for it if it was something they needed.
How they do it now I will let MR answer. It is worth pointing out that back when I started reading MR they'd review detail parts, paints and adhesives, decals, tools -- not just cars and locomotives and the odd structure (built up or kit) now and then as they do now.
Dave Nelson
I'm just going to take a stab at it, but from what I can see via reviewers (and not just model railroading, but in all other review subjects) in general; probably a bit of both. Some companies might ship product samples to a reviewer for free. Other reviews might be items that were bought off the shelf or ordered from the company. Sometimes if a reviewer gets a product for free they might mention it in their review with some disclaimer of the sort "I got this product for free but my opinions are still my own."
The reasons a company may wish to stop sending free product for a reviewer vary. For example if a reviewer requests say 100 samples of a product instead of the normal 1 or 2 free ones a company ships, that company might cut off the reviewer's benefits since they see them as attempting to exploit their system. If your asking for an entire unit train worth of freight cars for free as a reviewer, your bound to loose your friends with the manufacturers who sent you review samples to begin with. MRR only tends to review two or three products a month in the magazine. They might do more online, but their review output is pretty moderate compared to some of the busier reviewers out there.
How does MRR obtain a product to be reviewed? For example reviewing an Athearn HO scale SD40-2 locomotive, does MRR go to a dealer and randomly pick out the locomotive or does Athearn send them one that they selected? And is the item "free" or does MRR pay for it? and after the review, what happens to the item?