Table of Contents
Scratch-building an ARP Signal Box
Dave Coasby, Jeremy Everett & Mick Boyd - Luton MRC
Gazette - May 2018 (Vol 20 No.7)
Luton MRC member Mick Boyd first came up with the idea of us constructing an ARP (Air Raid Precautions) signal box for the club’s Central Works layout, as he felt “It would be something a bit different from the usual”.
The prototype
Construction of ARP signal boxes began in the late
1930s due to the threat of war and in particular of
aerial bombardment. There was an urgent need
for the railways to keep vital buildings operational.
Unlike traditional signal boxes that were usually
built of timber, ARP boxes were constructed with
thick brick and concrete walls and concrete roofs
and floors in the hope they would withstand bomb
blasts. They were built at various locations around
the country, in particular at vulnerable sites such
as strategic junctions, military and factory
locations, perfect for our project.
From the research we undertook there seemed
to be quite a variation in sizes and styles of these
boxes, though they all had a similar ‘look’ with the
brick and concrete construction.
Although we found several photographs of these structures we couldn’t locate any plans or any prototypes still existing anywhere nearby where we could take dimensions. The prototype that seemed to fit our requirements best was one situated at Town Green, Ormskirk, so this is the one on which we have based our model, though we did introduce a few changes as we went along.
Scaling from a photograph
As the construction of the box was going to be a team effort with Mick, Jeremy Everett and me, I thought I would draw a basic plan of all four elevations on my computer so that Mick and Jeremy could use the dimensions to help them cut out the walls etc. Fig 1 shows the front elevation. If you know the size of anything in a photograph you can usually scale everything else to it. In this case the obvious choice was to use the bricks. A brick with mortar between it is normally about 9in x 3in, so as I was just able see enough from the photographs to count the rows, both horizontally and vertically, I arrived at some reasonably accurate dimensions.
Windows
I produced a separate artwork, Fig 2, to represent the metal-framed windows and wooden doors that Russ, a modelling pal, kindly offered to laser cut from thin MDF. In the prototype photograph some of the windows were shown slid open. I quite liked the idea of this, so the open aperture was incorporated into the artwork for some of the windows. I also included a few spare frames just in case we messed up when it came to glazing. Jumping ahead slightly, once I received the cut out windows I lightly sprayed them with Halfords white primer. When dry they were lightly weathered as I realised it would be difficult to do this once the glazing was in place. Having purchased some ‘Glue ‘n’ Glaze’ and a sheet of clear styrene, it was time to start cutting the styrene to match the size of the frames and glue the pieces in place. My other purchase around that time was from Boots. This was a pair of white cotton medical gloves and I used these when handling the styrene to avoid marking it with my fingers.
The steps and balcony – Mk I
A look at the prototype shows these were made of pre-cast concrete. This method of construction
was quite common at various places around the country where concrete works would cast all sorts
of standard parts for the railway infrastructure.
Jeremy had a spare Skytrex concrete overbridge kit. Bingo! Some of the parts looked very similar to that of our prototype, so Jeremy set to on a bit of kit bashing and made up the steps and balcony, following the photographs as best he could. Unfortunately he forgot to check on my plans and made the balcony too long. But when Mick and I went to see it we realised we could quite easily extend the depth of the box, with an outside first floor WC that we’d seen on many other boxes.
The main structure
Mick was ‘volunteered’ to build the main shell and he intended hand cutting this from 2mm ply.
Fortunately around this time our friend Russ, who had use of his club’s laser cutter, said that as he was
about to cut out the windows, why don’t we let him cut out the main building shell at the same time?
Why not indeed. I quickly amended the artwork of the entire building to incorporate the
first floor WC and send him the vector file of the artwork by email.
So Mick’s job was made somewhat easier, having in effect a kit of parts to fit together without any cutting out. I breathed a sigh of relief when I realised my drawings must have been accurate, as I saw Mick glue all the part together without any problems.
The steps and balcony – Mk II
With the main structure assembled and the steps and balcony temporarily fitted, the signal box was
brought to the clubhouse for all the Luton MRC members to see. Although there was general
praise for our efforts so far it was felt the Skytrex parts looked too dominant against the box and
slightly over scale.
Back home I got my scrap styrene out. I measured the Skytrex pieces, looked very closely at the
photographs and started some modifications in earnest. I thought I could save some work by utilising the actual
steps from the Skytrex kit and glued some walls I’d cut out of thin styrene to them, adding a temporary balcony
to see how the whole thing looked against the signal box shell. I’m a great believer in ‘if it looks right, it
is right’. To this end I always have a 7mm scale figure standing near any models I build. It helps
me instantly judge if something is looking too big or too small. In this case even the modified steps
still looked too big. The resin steps, although going to more or less the right height, were definitely too
wide. So I cut about 5mm off the width with a saw and tacked the wall back on. That looked much better and with
the figure in place it all seemed to look right. Gradually the steps/balcony unit evolved into what you see in
the photographs and I was satisfied with my efforts.
There was general approval when I took the box back to the clubhouse where I managed to pass the project onto Jeremy for the next stage.
Constructing the roof
Although there are several photos of ARP boxes available, there are probably none showing the top
of the roof. The original would almost certainly have been one piece of cast concrete with raised
edges and a hole in one corner for drainage. Our model roof was cut from 6mm MDF and
overlapped the box walls by 7mm all round. Once again these measurements were estimated from
the photos we had. After careful measuring to ensure an even overlap, a piece of 2mm MDF was
added to the underside so that the roof could be accurately slotted into the open sides of the box
and allowing removal in the future. Half hexagon 7mm wooden strip was cut and glued to the top
edges and a hole drilled in one back corner for a drain and another for the stove chimney. The
whole was then treated with Polyfilla to ensure there were no cracks or joins in our one-piece roof.
Once set the whole was rubbed down ready for painting.
Three coats of matt emulsion, magnolia in this instance, sealed the surface which was then
treated to a concrete colour mix, painted by hand with a brush. Acrylic paint dries darker than when
applied, so allow for this in the mixing of colours. A mix of Tamiya matt white, buff and pale grey
seemed to work quite well to simulate concrete. Weathering of the whole box would take place at
the end of the build.
Careful positioning of the stove chimney is essential if it is to fit accurately over the stove that
was added to the interior later in the construction. The stove was to be fitted to the back wall, so after
measuring and measuring again the hole mentioned earlier was drilled and a chimney
created from 3mm aluminium tube. As the roof was to be a well for rainwater the chimney had to
be raised from the surface, so a small piece of thick styrene was cut and shaped to fit round the base
of the chimney. This too was painted concrete colour. A rummage through a large tin of 4mm aircraft bits and
pieces produced a ‘Chinese hat’ style cowl, which was glued to the top of the chimney and the
whole painted matt black.
Applying the brick cladding
It seems that the most common bricklaying pattern on British railways is
Dutch bond. A layer of stretchers
followed by a layer of headers. I am sure someone will disagree but that’s the pattern we
chose. The necessary sheets were obtained from South East Finecast. In order to get each wall cut
as one piece, accurate measurement is required. Each wall was measured for both vertical and
horizontal scale and two millimetres added to the horizontal scale to allow for butting up on the
measurements from the start and cutting with a scalpel type knife started. Measure, measure again
and then cut seemed to work quite well and we soon had four walls overlapping at each corner by 1mm.
Placing a block of wood along each edge to prevent flexing allowed the edges to be chamfered to 45 degrees using fine grade emery paper. Provided this is done slowly with constant checking, a reasonably accurate corner can be obtained. UHU seems to be the most satisfactory glue for attaching the plastic sheet to the wood carcase and eventually we had a brick patterned signal box. Basic colours are required before weathering so we started with the same concrete colour mix as used for the roof. Two thin washes are better than one thick one and so the total brick surface was included. We now had a matt surface of concrete colour.
the bricks red without that colour bleeding into the mortar courses. In this instance we chose Faber- Castell PITT artist pens, Indian Red 192. Working gently – with little pressure – we allowed the side of the pen to skim across the surface of the brickwork, leaving the mortar untouched. One pen was more than enough to give two coats of this Indian ink. Where there were the layers of engineering bricks, these were carefully painted over the red using suitably coloured enamel paint. The next step was to fit drainpipes and any other external pipework, like the soil pipework for the lavatory extension on the first floor. Scale Link can provide just about everything needed for such purposes. Having purchased our requirements we cut the pieces to size, filed to fit, primed and painted them matt black.
The interior
The Springside signal box interior kit contains considerably more than we could fit into our ‘box. The parts for level crossings and single line apparatus were put away, as they were not required. Making up the lever frames is time consuming and it does pay to prime all the parts and then paint them in the desired colours before assembling the frame. The shelf with signal bells is also fiddly, so once again we constructed and painted the whole before fitting inside the box. The floor had two coats of the nearest Humbrol tan we could find to represent linoleum and the walls had two coats of BR Stock Cream. When each piece of interior had been painted, it was glued inside the ‘box in the most appropriate and realistic position we could find. Only the aforementioned stove had a predetermined position against the back wall and immediately under the stove chimney. We perched an engine driver figure with right arm raised, in this instance reaching for the wall phone, on the stool provided and fitted him looking down or up the line from the side window.
Finishing the model
We decided to weather the whole building before fitting the windows as Dave had already weathered the window frames and we wanted to keep the ‘glass’ as clean as possible. Taking care not to get the weathering powder inside the box we gave all the brickwork and concrete a coat of Mig Industrial City dirt, brushing it into the crevices and working over the whole surface with a circular motion. It would be very easy to panic at this point as most of the red brickwork had almost disappeared. Carefully we wiped away the surface of the brickwork using nothing more than a finger. By continually wiping and cleaning said finger, the red brick returns, albeit now duller and dirtier than before. The roof and steps were similarly treated and in this case also had a wash of Modelmaster moss green to give the hint of moss growing on the concrete surface. The windows fitted perfectly and had this model not been intended to move around the country, we would have left them pressure fitted in the recesses. As it was a touch of PVA in each corner on the inside did the trick. The roof is detachable so that the interior can be viewed as and when required. Lighting has been added and in due course we will build some point rodding emanating from the front of the box and going off both up and down the line.
