North Pacific Coast Railroad
Building the North Pacific Coast Railroad
Timber Culvert Construction
by Ken Stanfield
David crosses a "timber" culvert on his Willamette
One of the interesting things about building outdoor railroads is dealing with the rain
and the resulting runoff.  Most of the time a simple piece of pipe under the track is all
that is necessary to insure good drainage.  However, I chose to make some of my
drainage culverts more interesting and more prototypical by building “timber” culverts.  
They are easy to build, robust and have a realistic appearance.  These culverts could
have been constructed of treated wood, but I used the same composite material that I
used for my ties – Trex.   Trex is a trademark of the Trex Company, Inc. Once installed,
they are virtually maintenance free. This is how I did it – keep the ideas you like,
discard the rest.

The basic design consists of 2x6s stacked to the desired height in 1 ½ inch increments.  
Measure or estimate the height to the bottom of the ties and divide by 1.5; round up to
the next whole number.  A 10 inch culvert will have  7 layers of material with the top
layer made with 2x2 Trex stringers to support the ties.
A word of caution.  Trex is not
recommended for structural members,
so I limited my spans to 24 inches.  The
strength of the stringer plus the strength of the rail supports the load without deflection
on my railroad.

Below the stringers, the layers are built from 2x6s that have been notched on each end.  
My top 2x6 layer is 6 inches longer than my ties.  Each successive layer is 3 inches
longer then the one above.    The result is an end support with a 45 degree slope.  (1 ½
inches out; 1 ½ inches down on each side)  By notching the ends, the bulk of the 2x6 is
hidden by ballast, leaving only the square 1 ½  inch ends exposed.  Thus the wall on
each side supports the track and retains the roadbed.
The basic pieces of a culvert: unassembled; assembled and installed.
The only “trick” in the construction  of the end walls is that the Trex tends to be convex
on one side.  I ran each piece on edge through my table saw to remove this crown.  With
2x6 material, I had to run it twice, once on each edge, to provide a flatter surface.  

Once the major pieces were cut, I used quality coated deck screws to secure each piece
to the next.  Start with the bottom two layers and work your way up.  I also used some
sealant between each layer to help prevent water infiltration.  The deck screws are the
weak link in this construction method.   They will rust off before the Trex deteriorates.  
Remember to leave space for a ½ inch hole on each side for a piece of rebar to anchor
the supports to the ground.   Drill a hole in the end of the top 2x6 on each side wall.  
These need to be loose holes so that the rebar does not disturb the end walls during
installation.   Once the two end walls are assembled,  turn them on their backs and
attach the decorative trestle bent to the face of each one.  Either 2x2s or tie material is
cut to length and each piece secured with two deck screws.  The outside bent legs are
sloped out at about 5 degrees.  I used a cross brace on the taller culvert.  While having
sides of equal height is convenient, there are times when unequal heights will be
Water drains through a culvert built six layers high.
Next, locate the stringers on the top layer and attach with deck screws.  For a 2 foot
span, I use two 35 inch stringers (5 ½ inches on overlap on each side). The outside
measurement should be about 9 ½  inches.  The stringers are the top layer of the side
walls.  Cut additional 2x2 material to fill in between the stringers and along each side
1 ½ inches wider than the ties.  These pieces will retain the ballast.  This completes
the illusion that the side walls are thin while giving ample support to the stringers and
track.  From the photos you can see that once the culvert is installed  the side walls
appear to be 1 ½ inches thick.

Installation can be handled in one of two ways – I have used both.  First, preinstall the
culvert and build the roadbed up on each side. This method requires an estimate of  
how high the culvert should be.  This works best on straight level sections of track
where a slight height miscalculation is not too important. The second method is to  
install the roadbed then dig out for the culvert.    This method works best on curves
and/or grades.  You have actual measurements to work from.  When I install another
culvert, this second method is  the method I will use.  
Method 1: Culvert before roadbed.
Method 2: Roadbed before culvert.
Carefully center both end walls on the right-of-way and check the distance between
them.  Mark the footprint and estimate the vertical distance that each side must be
lowered.  Next, excavate a shallow (about 2–3 inches deep ) trench below each wall.  
(Check again to make sure the footprint is right.)  Fill each trench with
dry concrete. (Be
sure your concrete has small rocks – big ones get in the way.)  Place the first wall and
level  as necessary.  Next, position the second wall and level as necessary.  Cross
check the distance between the walls, the square of the wall with the center line and the
level of the walls with each other and the roadbed.  Take your time – this step requires
a lot of fuss.  You may have to take the walls out and adjust the dry concrete several
times.  Be patient and do a good job.  Once you are satisfied with the result,  carefully
insert the four rebars and hammer them flush with the top 2x6 to pin the end walls in
position.  Be careful not to disturb the end walls during this process.  When the end
walls have been installed to your satisfaction,  gently water the dry concrete.  The
concrete will set up in time and provide a firm footing for your culvert walls.

With the walls and stringers in place, it is time to finish-grade the roadbed and install
track.  Again, I have done this two different way.  Method one, I prebuilt the deck with
the ties attached to the stringers.  Then I installed the stringers and deck as a unit.  The
ties are an integral part of the culvert structure.  Method two, I installed the bare
stringers then placed the track with ties installed on the rails.  This is now my preferred
method.  Long term maintenance is simpler and installation of the rail to the ties occurs
in the shop instead of the field.  Once the track is installed, a few deck screws through
the ties and into the stringers holds the track in place.  If fine adjustments are needed
later, use metal shims under the stringers to level the track.
Stringers with ties attached
Stringers without ties.
Troy takes his home built switcher over the new double culvert.
(The “wood” guard rails are not yet installed.)
A few notes about design variations.  The double culvert pictured above has standard
end walls as previously described.  The center section is a very solid support also
made from 2x6s.  The ends of each layer are U shaped.  Ballast is used to fill the ends
and give the appearance of a double wall.  The two foot span limitation could be
extended if steel stringers were substituted for the 2x2 Trex.  Attaching the ties to the
stringers would be a challenge but the result would be a deck girder bridge with timber
end walls.

Because the culverts are resting on concrete, over time the track will tend to settle a
little on each side.  On-going maintenance requires the addition of ballast and a little
track leveling.  The only other maintenance is weed control under the culverts.  These
“timber” culverts are a great addition to the North Pacific Coast Railroad.
For private use only – all rights reserved 2007
Be Safe.
If you attempt a project like this, you are responsible for calculating the
load handling ability of your culvert.  Failure of a culvert can result in
injury or death. The author and this website are not responsible for
checking your calculations or workmanship
Last update 07/20/07