What % to deduct for railway line?

[wowfudge]

We're going to get our house valued for sale and want to make sure we price it correctly. Trying to figure out ball park value ourselves first. How much would you deduct from a valuation for a railway line at the bottom of a long garden? Approx 5 trains an hour at peak times.

The reason I ask is because the more recent sales on the street have been of the houses with gardens backing onto the railway line, so I would expect ours to be worth more because our garden backs onto someone else's. Traditional 3 bed semi with gardens front and back plus garage.

I have lived in a few properties near to railway lines. One was a Victorian terrace approx 10m from the railway, and the other a modern 3 storey block of flats approx 15m from the London-Brighton line. It's surprising how quickly you acclimatise to being that close.