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Working Travelling Post Office (Alec K)

1 Background
1.1 I have found that my garden railway projects rarely have a single stimulus. In the case of the TPO for the Caradon Branch I can summarise the contributing factors as:
  • the re-issue of the 'OO' gauge Hornby operating TPO and the re-awakening of memories
  • embarking on a steep learning curve with garden railway electrics and electronics
  • my wife believing that her brother's Triang TPO was a better product
  • the need to add value to my modest garden railway
  • my son's enjoyment of his working spells at a Royal Mail Centre during his student days
1.2 The only disincentive was the lack of fidelity to a mineral railway prototype. Given my two years' experience to date with garden railway matters, readers are advised that what follows is a 'see-one-do-one-show-one' process. The 'see-one' stage was accomplished by courtesy of NHN's selfless disassembly of his own cherished Triang TPO with its detailed photographic record.
2 Donor vehicle
2.1 Newqida 45mm gauge European lwb goods van:
2.2 The van body is removed from its chassis, the press-fit ventilators and sliding doors being dispensed with. The top door runners are reattached to strengthen the body sides. Surplus handrails, air brake pipes and electrical connectors are removed, and artwork rubbed off with a draughtsman's eraser. The chassis has a new buffer beam fabricated and attached, the Newqida coupler extensions removed, and new LGB metal wheelsets added. The van body will have 'frosted glass' lights made from a hard plastic diary cover installed after repainting to represent the high level small windows fitted to pre-Grouping TPOs. 
3 Mechanism
3.1 The operating mechanisms for the on-train collection hopper and delivery arm ('Traductor arm' to give it the official GPO title) are both powered by servos supplied with the HiTec 2 channel AM radio control system purchased at a substantially reduced price from Howes of Oxford. At this point I should ensure that full credit is due to NHN, upon whose considerable R/C experience I was able to draw, and who became, effectively, a technical mentor for the project. His Triang TPO was also deployed to excellent effect!
3.3 The picture above shows, from L to R:
  • The receiver and servo driver unit. This was later surrounded by foam plastic padding to protect it from mechanical shock
  • The battery compartment. The battery pack is held in place with foam plastic padding.
  • The servo for the collection hopper, to which is fixed a thick plasticard cam bearing on a chrome steeple nut bolted through the left side of the hopper. As the servo rotates under r/c, the cam lifts the steeple nut in an arc. The hopper pivots on its mounting (visible in the picture) and extends out of the van side through the former door aperture to collect the mailbag from the trackside arm. The hopper returns to the vertical by gravity as the servo resets.
  • The traductor arm, made from laminations of thick plasticard with a reinforcing strip, and pivotted centrally on a brass rod drilled through the van floor and supported by a firework rocket nose cone. The peg on which the mail bag is suspended is made from biro pen innards. The arm is swung out of the door aperture by a plasticard cam bearing on another chrome steeple nut, and driven by the second servo. The return mechanism for the arm is a simple rubber band, attached to a metal clip bolted through the van side. The mailbag is scooped off the arm by a fixed trackside hopper.
  • The aerial, shown looped round the interior of the van. This is now a neater installation, secured with 'sticky fixers'.
4    Finishing
4.1 After extensive bench testing and adjustments, details were added to the van body to reflect the style of the elegant LNWR-built West Coast Joint Stock TPOs of the early 1900s. This view shows the addition of grab handles, steps and lamp backets from Brandbright and Garden Railway Specialists, and corridor connections made from thin plasticard and postcard. The Pre-Group and Grouping TPOs all had offset corridor connections to prevent access from the standard coaching stock to which they were often coupled. The heavily-moulded stanchions on the Newqida vehicle prevented me from reflecting this aspect; the mouldings add strength to the body ends and I was reluctant to remove them.
4.2 WCJS vehicles were finished in the classic 'Plum-and-Spilt-Milk' livery of the LNWR, a combination that is incredibly difficult to define. NHN supplied a colour picture of an IOM rake of coaching stock for reference, and I was able to match the 'Spilt Milk' shade from the Halfords range listed by the 16mm Society resource. The shade they recommended for the 'Plum' colour was not available and the nearest substitute was used instead.
With roof ventilators, vacuum brake telltales and reset valves, and bespoke artwork applied, here is the completed and working TPO. My other collaborator, Chris Moxham, was commissioned to design and produce the artwork and he has met his brief in full, creating the 'South West Coast Joint Stock' branding - we wished to cause no offence to LNWR adherents on the grounds of inaccurate colour matching! - complete with a coat of arms that carry a medieval ship flying the flag of St Piran and an authentic GR mailbox with the 'halfpenny stamp' premium notice. The flying lead visible on the extreme right of the TPO connects two LGB coach lighting sets inside with the socket on a LGB Mail tender van that has wheel pickups.
4.3 The trackside apparatus tries to reflect prototype practice but to replicate the netting of the collection apparatus was a step too far at this stage.
The GPO shelter (and the 1930s-uniformed postman) are reasonably accurate, as is the arm on which the mailbag is suspended. In reality, this was swung out at 90 degrees around 10 minutes before the TPO was due to pass, and it certainly didn't have a GWR-style finial. The collection hopper is a pragmatic plasticard structure which will need to be redesigned as it fouls the LGB mail tender. The rear of one of the pair of yellow and black chequered clearance warning boards can be seen in the left middle distance.
5 Acknowledgements
Thanks are due in full to NHN and to Chris Moxham for their enthusiastic support and advice.
An Illustrated History of the Travelling Post Office by Peter Johnson, OPC, 2009 is the principal source of technical background - and a fascinating story as well.
6  Amendments and Modifications
6.1    The trackside collection hopper design has been scrapped and a new design trialled to deal with the clearance problems caused to other rolling stock. The base unit has been moved further from the running line to give the standard clearance for the Caradon Branch.
6.2    The trial version allows the hopper to swing through 90 degrees such that its 'mouth' is parallel to the TPO side and at at a height that scoops the 'mailbag' from the traductor arm. The 'mailbag' is seen here having been collected under radio control (and is self-evidently a 11/16ths thread nut slung from copper wire).
6.3    Here the hopper has been swung back to its 'parked' position clear of the running lines:
6.4    This now means that an element of manual intervention is needed to arrange the collection of mail, entirely in accord with real practice of course! The r/c trials this afternoon have proved satisfactory, although it has been a little tricky pushing the TPO with one hand while simultaneously operating the r/c transmitter with the other.
6.5    I have added what I hope is a rather better picture of the TPO than that used above. The traductor arm is seen in its stowed position.
6.6    Modifications up-to-date as at 18.00, Saturday 26 March 2011.