I very much hear what you‘re saying about wood Simon, as well as the challenge of your location by the sea - but it might be worth reconsidering, depending on how long you want the new line to last and whether the techniques used on your late friend’s line could be changed. At my last house, the slope of the garden meant that half of the line was above ground and half was at, or slightly below, ground. For the above ground section, I used met posts to support the tanalised uprights and prevent any part of the wood touching the soil, with treated decking boards on top, strengthened at the edges by cheap fence capping strips. I didn’t use any kind of roofing felt on the boards - the ballast was simply loose on top. I gave the line no further treatment and when we moved after a decade and I dismantled the raised section, there was no rot anywhere. Rightly or wrongly I’ve put this down to the prevailing south-westerly wind which blew straight down the garden and dried everything in its path - soil, pots, clothes on the washing line and my railway. My wife has always worked on the basis that many things (cuts, grazes, blisters, drying washing, gossip, etc) are much best dealt with by ‘letting the air get to it’, and (barring any other suggestion), I think this is probably the most likely ‘Occam’s Razor‘ explanation here.
Our new home was more of a challenge, as the only space for a railway was at the bottom of the garden, within 6 feet of a stream that regularly overflows in winter, so the basic soil and air conditions are pretty damp. However, the prevailing wind is still south-westerly, so I thought I’d take a punt on using the same basic construction techniques, but modified to build in extra steps to avoid moisture traps. So far the main section of the line has been in place for 8 years and I haven’t yet found any signs of damp in either the posts or the boards. The changes I made were a) to impregnate all cut ends on the tanalised uprights with cheap superglue, b) to replace the upright-to-baseboard butt joints with inverted plastic decking feet, which had the benefit of giving me the ability to adjust levels precisely, and c) using galvanised washers to create air gaps between the plastic decking feet and the boards above. I was worried that the torrential rain of the last three winters would have done serious damage to the construction, but so far the only casualty has been endlessly disappearing ballast (mainly due to heavy rain, but in part down to my hens who like to use it to top up their crops).
There’s a couple of downsides to all this - firstly the boards seem to need one full season to settle down properly, but once they do, the trackbed has been completely stable, taking the 10-12kg weight of my Earl of Merioneth or my NGG16 and the same weight again in carriages. On one occasion, the full length of the line supported the weight of a fast-moving mature muntjac with no problems or damage. And secondly, while the regular expansion and contraction of the wood during wet and dry spells effectively makes no difference to the running of the trains (because, I think, the tolerances in the plastic decking feet allow for some flex), it’s a complete nightmare if you want to incorporate a removable section or a lifting bridge.
Here’s a few gratuitous photos:
And lastly a photo from my previous line showing how lonicera nitida covers a multitude of my constructional sins (while still ‘letting the air get to it’):
