Boy if there ever was a purely personal, and widely variable reaction, this is it. The fact is lots of people live closer than that (300 ft = football field) to train tracks, freways, oil refineries, hog farms, lumber mills and all manner of things that make noise or smell bad. My bedroom window was once 25 feet from my neighbor's house where their 8th grader practice playing drums every night from 10:30-11:00, but somehow I managed to get to sleep. That same bedroom was probably 500 feet from train tracks and I almost never heard the trains. T
Today I live in a townhouse that's probably 50 yards from a busy street. We hear traffic noise all the time. But it's also an end unit, at the end of the complex, in the middle of many mature trees, overlooking the creek and had a kitchen that made my excellent cook wife say when she first saw it, "This is my kitchen/" So on balance, we love our place because all those other things (and mored) outweighed the negative of traffic noise.
But your question came to how that sort of thing affects the value. I'd say the effect on the number of potential buyers is greater than the effect on value. Some percentage of buyers refuse to consider anything close to any noise or smell, but that's not the majority. There's no perfect house. All buyers make trade offs, and train tracks are just one for your house (I assume you're a seller). There's no formula for how something like proximity to train tracks affects value.
I'm sorry there's not a simpler answer, but that's real estate.