Drewry 'Twin-Set' Railcar (Fiat)

How I came to make this model.

At the time of joining the Christchurch Garden Railway Society, I did not possess any      G gauge trains, but I saw it as an opportunity to build my own. I visited the library looking for something about New Zealand railways, and came away with a book describing the Drewry rail car. It included a basic plan, elevations and a number of photographs. Thus started what was going to be a long journey.

There were 35 of these built, nos. RM100 – RM134 and the first went into service in New Zealand in 1955. The last of them endured until 1978.

 I chose RM133, as it was based in Auckland, Wellington and Christchurch during its lifetime and was one of the last to be in service.

 At 1:24 scale (1 3/4” = 3’-6” gauge) it is 1.35m long overall.

Getting started.

As seen in the drawing programme.

Using my cad software, I imported scanned images from the book, of the elevations and plan. By tracing over the images I produced a line-work version of the rail car. This was then scaled within the drawing to make it the correct size for the scale of the final model. I had major dimensions (bogie centres etc) available to be able to do this. I was then able to determine other dimensions too.

A very early stage.

The wheels/axles are ‘off the shelf’ at very close to scale size. The chassis is constructed from wood. I have used styrene plastic sheet for the body, 2mm thick for the sides which were routed out for the windows. The roof and lower sides (tumble home) are from 0.5mm. The vacuum formed ends are from 0.75mm. The two cow catchers, six step assemblies, the 21 air vents and six bogie sides are also all vacuum formed.

The model is battery powered using NICADs with an RCS controller unit. One end bogie is powered, via a ‘Tamiya’ gear train connected to a Decaperm motor above the floor.

I am now using 2.4G radio, having ‘given away’ 27Mhz and all of its interference problems.

The one driving bogie.

The reduction gear and Decaperm motor.

The powered bogie uses Tamiya gearing, with an overall gear ratio of 1 to 1.
This is connected to the gear train by a floating axle assembly. The pivot point of the bogie coincides with the centre of one of the ball joints on this shaft, allowing the bogie to turn and be powered. The gear train is then connected to the Decaperm motor. I am using the gear reduced end of this too.

The control end.

An RCS module gives forward/reverse/ramped speed control, start/stop of a diesel sound generator (pitch changing with speed) and triggering of a horn. This is a digital storage device loaded with an air horn noise down-loaded from the web. There are two speakers for this, one located behind each cow catcher. Only the appropriate forward facing one is enabled for each direction of travel. One of the relays controls this feature.  My interface board provides 'open collector' transister outputs turned on by the control module. The radio receiver (Rx) is only the size of a postage stamp and is supported by a piece of foam rubber. Its power is provided by the control module through one of the channel outputs.

A variety of plug/socket connectors are used between the two halves. The number of these grew as the project developed.

The power end.

10 x 5000mAh D size NICAD batteries provide the power which lasts for a number of hours. There is a battery recharging socket and the main ON/OFF toggle switch protrudes below the floor level. I just have to remember where it is on the model.🤨 Power to the LED lamps in each body section is provided through a connector located in the appropriate chasssis section.

My router set up to create the window openings.

After the first cut only. Undersized.

One completed section of windows.

The styrene sheet must be VERY well clamped in position while using the router. I found it best to do the routing in two stages. For the first run I kept the routing bit about 2mm off the guiding frame. The styrene tends to literally go to custard when fully enclosing the bit. I then did the final cut, moving the router in a clockwise direction, around the frame and a clean cut was achieved. The 1/4" bit that came with the router was the right size for the window corners.🙂

The very first two frames being glued together.

The front horn speaker can be seen here.

A typical frame.

The internal frames are cut out of 1.5mm styrene and are all the same shape. I routed out the lower rectangular recesses for consistency. These fit over the chassis sides. Having cut the shapes out roughly, they were stacked and held together with temporary screws. Using a printed out paper profile stuck to the top surface of the stack I proceeded to use my disc sander to trim them as a whole to the required size.

The vero board behind the LEDs.

One of the vacuum formed ends being glued on.

LEDs, super bright and one modified in my lathe.

The red LED lights were a bit of a mission. I used standard 5mm diameter coloured ones which I already had, and turned down their ends to the required size (4mm) to fit into the lamp protrusions. These turned out to be nowhere bright enough out in the daylight. More super bright LEDs (which I was using for the four head lamps), though this time they actually glow RED, were then purchased and the replacements were made. Because the bright LEDs are clear, I tinted their ends with a red felt pen. The final result was very successful.

The illumination of the white and red lamps, indicating direction of travel, is swapped over by a relay in association with the reversing of the motor.

The bogie side plug was made from bits and pieces. Note the holes in the base board to aid in the evacuation of the air.

The basic painting of the model was done using an air brush and fast dry enamel suitably thinned for the process.
The colour is Insignia Red, the only red available in the range.
The black and white is achieved by carefully applying sticky backed vinyl artwork.
The actual shapes were drawn in my draughting programe at real size.
The curvy end shape was printed out on paper first and held onto the model to check size and fit.
Some adjustments had to be made to the drawing to get it right at the front corners.

 The final drawing was saved as a .dxf file and taken to a sign maker. They printed it black onto white vinyl.
I had to cut out the shapes using a VERY sharp blade. This included the round holes for the tail lights. Because the backing sheet was white, and has thickness, I carefully went around all of the edges with a black felt pen before placement on the model.
Following a tip from the sign maker, I used a very dilute dish-wash liquid solution as a lubricant to aid the placing and alignment of the pieces. Without this they would stick immediately, probably in the wrong place. Dabbing with a tissue soon expelled the water.
Yes, it was fiddly cutting and positioning the pieces inside all of the door recesses, x 8.

I still have to produce and position lettering on the model.

Because I had no intention of making any internal details, the windows are blacked out. I used clear styrene sheet with black felt tip pen ink on the inside surface. The horizontal glazing bar across the main windows is a length of 3mm, chrome, striping tape stuck on the outside. The windows within the cab ends are similar, but cut to size to fit into the window recesses in the moulded shape.

The couplers are cut and filed from MDF board.

Oops, the orange peel paint finish was not intentional.😠

I should have dusted it before taking the photo.

The two air horns are turned from brass rod using my Unimat 3 modellers lathe. A brass strip soft-soldered on, attaches them into the end mouldings. I used a file as a tool to achieve the curved/tapered shape😮. The second one was made immediately after so as to get them as alike as possible.

The ‘glass’ (actually plastic) of the four head lamps was also turned by gluing a roughed out circle onto the end of a piece of tube with PVA glue. The glue had to be very well cured to be able to stand the stresses of this process. So I found out the hard way! A narrow strip of 0.5mm styrene was then super glued around the circumference. The front edge was then trimmed back in the lathe. I painted the rim while it was still held in the chuck, by rotating it by hand and holding the paint brush steady on the tool post. Finally I soaked the supporting tube off in water.

And one final thing..........

The extra toggle switch is on the right side and the push button is lower left.

I had some issues with the 2.4Ghz radio control set that I purchased to control the Fiat.

It only has a short range, I found this out when trying it in a model yacht. This however should not be a problem in a model train situation.

I did discover (and subsequently found reference to it on the web) that the servo throws are intentionally made smaller than is more normal. With a mechanical linkage this may not be a problem, however when the Rx is connected to an electronic circuit, it can be.

The extra wires added.

The RCS module provides for ramped speed control. Partial stick movement off centre provides a SLOW speed change. Full stick movement should initiate a FAST speed change. In my case however the fast option was not being selected. The easiest solution for me was to fit a, centre off, three position toggle switch to the Tx case.

This then gives the fast change, which can be used in an emergency stop.

I had a problem as well with the triggering of the engine sound unit. It so happens that when you program the RCS module for a latched output, as opposed to a momentary one, it requires more ‘servo throw’ than the Tx provided, to set the output to ON or OFF.

To fix this problem I high jacked the Tx push button switch which was there to select a buddy box. This button now gives me engine sound ON or OFF, and works well.

Go to My Train Layout to see a video clip.

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russell sullivan 26.02.2018 08:23

this is a beautiful model. I first saw a ststic DM model at Napiers Lilliputt model exhibition aged 7 and always remembeted it. Congratulations I love your wor