When i double-dug the beds i found it really difficult to fork over the sub-soil. It created a scraping noise on the fork and i had to jump on the fork to get it in
This sounds a bit like our plots, the surface water runs off the hill on the top of a rock bed under the topsoil.
The rock tends to undulate and form hollows and these 'pond' up and fill with surface water. If you have an indentation under your plot you suffer water problems if you have a hump under it, you are relatively free of problems.
Like you we have a disused railway line up hill of our plots which in pre Beeching days was well maintained but after his action is was not, hence our problems.
To try and over come the problem I dug a trench down to the rock on the 'uphill' elevation of the plot, and down the lower side of the plot.
Then I scrounged pipes of any description from demolition sites etc and placed them in the bottom of the trench. This took away the heaviest of the flow meaning my plot dried out much quicker.
In the end all the plot holders got together a dug one long ditch along the upside of the plots and directed the flow into a culvert we found.
I am pleased to say this has worked and the plots are now more or less free of flood water but it was 'bl***y hard work getting it that way.
I only thought I would mention all this because your predicament sounds like the one we had.
Best of luck in what ever you do!!
