The above photo shows the "HO" Plymouth with an S scale F unit.
How close is this to correct S scale?
There are many prototype Plymouth photos at:
The ones that most closely resemble this unit are the DDT 12 ton units.
Note how the width of the unit is almost the same as the track gauge?
This is a very small unit, the model is probably a bit large for correct S scale.
I found dimensional data for one unit that looks VERY similar to this "HO" model;
the ratio of hood width to cab width is less on the prototype.
Here are the S scale dimensions of the model compared to the above unit.