Bridge construction

I decided to add a low 1.7m long lift off bridge on the extension loop for three reasons - to provide access for the ride on mower, to avoid having to build an embankment at the lowest part of the line, and because I like bridges! A standard 2.5m long curve was placed over the gap to determine exactly where the rail should be cut for the bridge section. I did this symmetrically to provide equal short approach tracks at either side.
The three 25mm square cross section steel tubes were bolted to the track panel using 8mm bolts. The rails are always (just) within the two outer tubes throughout the curve to ensure stability.
The bridge supports were each made of two pieces of 60mm by 45mm weatherproofed timber, well soaked in creosote. These were glued and screwed together with a 25mm height difference, so that the approach sleeper could be fixed to the higher part, and the bridge dropped on the lower. The basic bridge (here with just two of its three steel tubes) was placed to allow the position of bottom of the supports to be marked on the adjacent concrete edging.
The track bed was removed down to the road core, and this was well compacted.
This was covered with a layer of mortar.
A 40cm by 40cm concrete paving slab was placed on this to provide a firm foundation, its height aligned to the previously drawn marks on the edging. The wooden support was then firmly bedded onto this with more mortar.
Once the mortar had set, the hole was filled with ballast and the approach track fixed in place, the last sleeper being attached to the wooden support with screws. The second support was added in the same way. The bridge was repeatedly placed over the gap while aligning the second support to ensure that the bridge track gradient and camber matched the approach tracks exactly.
With the bridge in place, two small guide blocks of wood were screwed on each support so that the bridge could be lifted on and aligned securely with the track. The square cross section steel tubes were painted with two thick coats of anti rust primer, and two top coats of matt black.
Finally a central pier was added to minimise the strain on the bridge with a fully loaded train. A small concrete block was sunk into the grass on a bed of road core. The mower can ride over this. A removable piece of 50mm wide creosoted timber was cut to size and positioned between this and the bridge, held in place by a couple of small guide blocks.
The bridge is light enough to be easily movable by one person, yet strong enough to support a loco and wagon loaded with three adults.
After five years, the concrete edging by the bridge supports had splayed outwards. I added some extra concrete around the wooden supports to help maintain track alignment which was still spot on.