S scale models will fit best on a standard 1-inch grid, and with your heroic scale 25mm miniatures.
With a 1-inch grid, where each 1-inch square represents a 5-foot square, you are playing at a 1:60 ratio (where 1 real-life inch represents 60 in-game inches.) Your closest model railroad scales are: (source)
- S scale (aka 3/16" scale) at 1:64, or ≈7% smaller
- O scale (aka 1/4" scale) at 1:48, or ≈25% larger
- OO scale (aka 4mm scale) at 1:76.2, or ≈27% smaller
- HO scale (aka 3.5mm scale) at 1:87.1, or ≈45% smaller
If you're looking for items that vary largely in size (trees, rocks, etc.) then any of these scales should look natural. (In fact some sellers lump S&O scales together for such items.) If you're looking for something with a much more defined size, such as a bed or a table, you'll probably want to stick with S scale.
A note on miniature sizes: the 28mm scale (aka heroic scale 25mm) refers to "the approximate height of a humanoid figure from base of foot to eyeline." (source, source) Figures tend to vary, however, and historically have leaned on the larger side of the scale, but should remain close to the 1:60 ratio of the 1-inch grid. (source)