Question: what are the different size model trains?
Answer: no letter = standard gauge (prototype: 1,250–1,700 mm or 49.2–66.9 in)
m = metre gauge (prototype: 850–1,250 mm or 33.5–49.2 in)
e = narrow gauge (prototype: 650–850 mm or 25.6–33.5 in)
i = industrial (prototype: 400–650 mm or 15.7–25.6 in)
p = park railway (prototype: 300–400 mm or 11.8–15.7 in)