The most delayed post on this blog (so far) I meant to write this 8 years ago, it was a planned 2-parter with Building Bridges (1) describing two alternate styles of removable bridge. The second bridge itself was complete before I’d written up the first!
Since then the plan for this post has gone through 3 iterations: originally intended to be a quick summary of the key design points, within a year or so it had become clear that I shouldn’t have built either bridge out of wood, as expansion and contraction in the wet and dry weather meant the bridges didn’t just drop or swing into place. This post was going to be a ‘how not to do it’.
Then lockdown came and I just wanted to run trains. And I realised that spending a minute or two each running session was actually much quicker than building new (possibly aluminium) bridges, especially in a situation where materials were not readily available! The bridges got a reprise, and still function today.
Building Bridges 1 described a very simple lifting bridge, bridge number 2 at around waist height is longer – it could have been a lifting bridge too but it would have lifted high, and swinging it seemed like it would be easier to do. So the design is modelled on a gate – I even originally used a gate latch.
In July 2016 I sunk the posts in exactly the same way as for the first bridge.
I thought of it as a gate. I put a post down the hinged side of the ‘gate’ and braced the ‘top’ of the gate, which of course was the trackbed, with a crosspiece. The rest of the gate I missed out…
I put a gate latch on to keep it closed, but also to hold it open.
This turned out to be a poor design choice, providing no precision in vertically or horizontally aligning the track.
I angled the trackbed end, so that the corners of the trackbed ends on the bridge and the abutment would not foul each other.
This was a better design choice, enabling me (with a bit of force) to swing the gate into position in all conditions. The gate latch has long gone, replaced with a washer underneath to align horizontally, and a bolt to hold the gate shut.
My ultimate plan is still to replace the bridge, probably with something in aluminium, at least for the longitudinal supports. Shortly after building these bridge I was involved in assisting with constructing the raised level garden railway at Cardiff MES and we created bridges with aluminium bearers parallel to the track and wooden cross pieces. They are light and fit perfectly into place every time.
But the urgency to replace my bridges has gone. Trains make it across the joins (just) and other parts of the trackbed are in more urgent need of upgrade (another story). I might even print some cosmetic rails and girders to make the bridges look more scenic, until the big reconstruction…
