Hi BS, and welcome to the forum.
So if I understand correctly, you are refurbishing the house in order to sell it and you are wondering if putting in a viewing window which wll give good views of the railway would enhance the sale ?
Well if it were my project I would be fundamentally interested in whether doing so increased the value of the property by more than what it costs. Although Estate Agents are viewed as objects of hate by many people, I've met some who really knew their subject. My suggestion would be to bring in a few rival agents and get their opinions. My guess is that they will say that you probably won't see a return on the money, though it would probably make the house more 'marketable'.
IME, when it comes to asking people you know....many people will tell you that you have a great idea and be very enthusiastic for you to spend your money - but when you actually ask them if they would like to buy it they all stare at their boots.
Or I could be completely wrong - it happens regularly.
Anyhow, I'm sure others on her will share their thoughts with you - in fact you could even set up a poll if you like to get an idea of peoples' feelings.
And make sure you let us know how you get on.
Best Wishes, John