How I constructed a small wooden trestle bridge

The bridge leading to the mill siding (see How I constructed the mill siding) was, like the swing bridge (see How I constructed the swing bridge) and two of the locomotives (see How I constructed an 0-6-2T locomotive and How I constructed a 2-4-2T locomotive), inspired by a similar structure on the Southwold Railway. The Southwold original was adjacent to a water mill and so seemed appropriate for my mill siding.

GER drawing of the wooden trestle bridge on the Southwold Railway

The raw material for the bridge was purchased from Wood Supplies, who specialise in supplying wood sections for model makers. Initially, I calculated the scale dimensions of all the timber sections but realised this was making the order far more complicated (and expensive) than was necessary and so I eventually adopted my key principle of over-engineering structures and opted for two sizes of oak timber section -  ½" x ½" and  ½" x ¼" - which were supplied in 18" (450mm) lengths.

Two of the  ½" x ½" timbers were marked out with the positions of the supports. These were chosen quite arbitrarily to coincide with suitable rocky locations on the stream banks for the support footings.

The beams were then positioned beneath the rails and the lengths of the supports marked-out on another piece of ½" x ½" timber.

The sandstone beneath the footings for some of the supports was chiselled out to improve their seating.

The length of each support timber was then shortened by ¼" (the thickness of the narrower timbers which will act as sleepers across the bridge), and sawn off.

 The supports were checked again, in situ and slight adjustments made to ensure they were the right length and shape to fit their locations.

The supports were then glued to the longitudinal beams with exterior grade PVA and held in place with clamps until the glue had entirely set.

The assembly was then placed in position to check clearances.

The central cross-members were then cut to length (85mm) and glued in place. Their location was dictated by the length of the shortest central support timber - the reasoning being that the diagonal bracing timbers would sit at an angle of 45 degrees and hence the horizontal distance along the beam would be the same as the vertical distance for which the brace would provide the hypotenuse (see photo below: X = Y )

Isn't it amazing how many academic disciplines are involved in garden railway modelling? Weights were placed on the timbers while the glue set.

When the glue had set, the length of the diagonals were marked out, using the shortest vertical as the datum .......

..... and the four diagonal braces were cut out from a piece of ½" x ½" timber .....

..... before being glued into placed between the central supports and the cross-members. A further piece of ½" x ½" timber was cut to fit between the cross members and glued into place.

Seven short lengths of ½" x ½" timber were cut out 35mm in length, tapering to 25mm .......

...... and glued behind each of the vertical supports just beneath the longitudinal beams. The eighth was omitted because there was insufficient clearance between the beam and the rock.

Twenty sleepers, 90mm in length, were then marked-out on the ½" x ¼" timber .......

.... and were cut-out. Initially, these were marked at 49mm centres for the track, but later I realised that, as the track over the bridge was slightly curved, I would have to mark the track centres when the bridge was in situ beneath the track.

 These were then glued into place on the longitudinal beams, one sleeper width apart.

 Two sleepers were then trimmed down to 85mm and glued to the central supports, behind the diagonals and beneath the beams.

Once the glue had dried ........

..... the bridge was slotted into place .......

...... and tested, with rolling stock .......

..... and with running water.

At present, the rails have not been fixed to the bridge and the bridge has not been cemented or glued into place in the stream as I want it to remain removable until I have decided whether to add further details. These could take the form of cosmetic brass pins at the joints in the timbers, track spikes and sealing the bridge with stained wood preservative.

Although the original bridge on the Southwold Railway did not include check-rails, I am considering adding them to my bridge as I feel they would add to the bridge's appearance.