Hello Kelly and thanks for your question.
If the home is located 100 feet from the train, then the sound of noise becomes a very distinct problem for many buyers, thus reducing the number of people interested in buying the home. You can obtain a train schedule for any track to determine when the train passes. However a 1/2 mile distance from the train (or about 2600 linear feet) is rather distant, and many individuals don't find the positioning of the train tracks 1/2 mile from the home as disagreeable as right next to the home. So as to the train tracks presenting to you, as a buyer, a "bargaining chip" in negotiating the price, the answer is that it would depend entirely upon the closeness home to the train tracks and the frequency of the train movement along the tracks.
I once rented a home that was located 200 feet from a commuter train line that ran from 6 a.m. to 9 p.m. (all day) with trains leaving the station about every 20 minutes at the peak traffic hours. I thought I'd never get used to the sound, but 30 days later, I barely heard it. After 6 months, I never heard the train again, and yet the train never stopped running. I just became rather "immune" to the sounds. So, much like motion sickness, I do believe that most people can get used to noise.
Hope this helps answer your questions!
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA