If you're comparing 1.435 * 10^(-3) mm to 1.435 * 10^3 mm, then the more reasonable measurement is 1.435 * 10^3 mm since,
1.435 * 10^3 mm = 1435 mm = 1.435 meters
1 meter is about 3.28 feet approximately
So 1.435 meters is slightly larger (about 4.708 feet) which is a fairly reasonable distance between the tracks on a railroad.
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Something like 1.435 * 10^(-3) mm is equal to 0.001435 mm, which is far less than 1 mm.
