Hi Steven,
I have lived in two houses that backed onto the railway. One at 22m, on a moderately busy non-electrified double-track mainline, and the other 200m from the West Coast Main Line - the same line as your proposed purchase.
At 22m you can feel every train that passes, let alone hear them. I lived there for 10 years.
At 200m you certainly hear them, though of course some are more noisy than others. I lived there for 2 years.
You do eventually get used to them and at times I wouldn't have been able to tell you when the last train had passed.
Passenger stock tends to be reasonably quiet. Higher speed trains can cause a pressure wave to hit the house, even at 75mph.
What surprised me the most when I moved to the second house was the noise of electric-hauled freight. I thought it would be quiet. It isn't. And there's a lot of it on the WCML.
The latter house, also a new build, had noise-proofing stipulations in the planning permission but they weren't worth the paper they were written on, in my opinion.
In the summer, when it's hot - like last week - most people are going to want to keep the windows open day and night. That's not going to be much fun and is likely to ruin your night's sleep.
The other thing to think of is track maintenance. This tends to happen at night, over weekends, bank holidays and Christmas. You will, occasionally, have noisy works with plenty of lights going on day and night right outside your house.
And then add in the flood risk, as pointed out by P Binnersley above - I checked on the Environment Agency flood map to confirm. In my view, this doesn't seem like a prime piece of real estate.
As Brissle Girl wisely said earlier in this thread "think very carefully before going ahead."
Would I buy it? No way.
As a new user I'm not sure if you can DM, but if you can, feel free if you need more info.