St Petrock goes AC

Once upon a long time ago I was privileged to be a regular operator on Peter Denny's famous Buckingham model railway. My station for the best part of 37 years was Grandborough Junction, where one of my important jobs was to communicate with the signal box up the line at Quainton Road... except that there wasn't a Quainton Road on the model.  To simulate this station, Peter built an amazing electro-mechanical computer that ran the timetable, controlled the clock, set points in the storage sidings and communicated with the 'up' block instrument at Grandborough.

Hitherto, the 'Quainton Road' storage sidings had been operated by Peter's youngest son Crispin, but sons don't stay at home for ever. The computer that replaced him was therefore known as the Automatic Crispin. Peter wrote an article about it for the Railway Modeller and I well remember the title  - "Buckingham Goes AC".

It will therefore come as little surprise to friends and followers of the St Petrock saga that my station should also have a computer to simulate the signal box up the line. However, time and technology move on, so rather than building it from plywood and Meccano, I've used a Raspberry Pi single-board computer. In honour of its illustrious ancestry, though, it too is known as the Automatic Crispin.

Mine has a slightly simpler job to do as I don't have points on my storage sidings, but I did want it to display operational notes and the timetable for the St Petrock operator.

My first task was to learn how to instruct the Raspberry Pi to perform to my wishes, so I spent several weeks teaching myself Python, then writing a program.  Fortunately, Python has some similar features to the Control Basic language that I used to use at my work, so the task wasn't too daunting.  I also discovered that program code for many of the routines was available on the Internet.

Here's a little bit of the code that checks the St Petrock bell button and trip rail near the storage sidings, and displays the 'hours' value. It would probably turn expert programmers to drink, but it works and I'm actually rather proud of it.

def get_button_push(after_push_time):
    GPIO.output(12, GPIO.HIGH)
    while True:
        if (GPIO.input(20) == GPIO.HIGH):
            time.sleep(0.1)
        else:
            GPIO.output(12, GPIO.LOW)
            time.sleep(after_push_time)
            return()
def get_storage_exit():
    while True:
        if (GPIO.input(16) == GPIO.HIGH):
            time.sleep(0.1)
        else:
            time.sleep(2)
            return()

def display_clock(hour,clockhr,minute):
    if (minute == 0):
        rect = Rectangle(Point(377,10), Point(417,50))
        rect.setFill('black')
        rect.draw(win)
    if (clockhr)>9:
        txt = Text(Point(397,30),clockhr)
    else:
        txt = Text(Point(407,30),clockhr)
       
    txt.setTextColor('white')
    txt.setSize(30)
    txt.draw(win)

The screen is an Elecrow 5 inch LCD display that plugs directly into the Raspberry Pi and sits on top of it. Here's a close-up.  You'll see that the operator has a couple of instructions to carry out before train working No.2 can depart for Tregarrick North at 6.40am.

Now the M7 has been attached to its auto coaches and waits at platform 2. A light beneath the bell-push reminds the operator that the clock has stopped  – trains at St Petrock never run late! After checking that the rotating storage sidings are set to road 5, the operator presses the bell once (call attention). The signalman at Tolcarne Junction responds with one beat on the bell.

The St Petrock signalman (or frequently signalwoman) sets the points, then pulls off signals 15 (platform starter) and 12 (advanced starter). The Gas Street crossing keeper also obediently opens the gates.

St Petrock now rings '3 pause 1' – 'Is line clear for a stopping passenger train?' Tolcarne Junction responds by ringing 3-1 and turning the block instrument to 'LINE CLEAR' (green).

The M7 sets off with its train, past the signal box. (Note to self: "That box needs a name board and some levers!")

As the train passes the signal box, the signalman observes that it has a tail lamp (and therefore hasn't left its tail behind) and rings 2 bells (ding...ding... train entering section). Tolcarne Juntion acknowledges with 2 bells and turns the block instrument to 'TRAIN ON LINE' (red).

As the train enters the storage sidings it crosses this trip point (two breaks in the nearside rail) that momentarily energise a relay, instructing the Automatic Crispin to ring '2 pause 1' (train out of section).

St Petrock acknowleges with 2-1, which restarts the clock. The Gas Street crossing gates close once more and the sequence is complete.

Gas Street Level Crossing

I was determined to have an operational level crossing on my railway. As it's turned out, the crossing gates control a very minor road indeed, just the access to St Petrock gas works, but they do add operational interest and keep visitors enthralled. Crossing keeper Fred Cockram and his long-suffering wife are kept fully occupied and the couple have been rewarded with a nice house to live in.

It's recently dawned on me that Fred ought also to have a little shed by his crossing, from which to communicate with the signal man at St Petrock, so this will be added before too long.

The crossing gates are from Peco and they're operated by a pair of Peco Smartswitch servos. The challenge, of course, was to get the gates to open and close in sequence and this was achieved with a couple of micro-switches, activated by the servo arms.

In this photo the bottom left micro-switch is being held closed, so when the crossing is activated the right hand servo will move first. When it has completed its travel, it will close the top micro-switch and activate the other servo. The amount by which the servos move, and their speed, is controlled by the Smartswitch control module (on the right).


Here is a video of the crossing in operation...

Gas Works

Gas works were a common sight until the advent of gas supplies from the North Sea around 1970 and they usually had rail connexions for the delivery of coal and removal of coke.

St Petrock station draws much of its inspiration from Bude, which had a large, relatively modern gas works. However, I preferred the somewhat ramshackle appearance of earlier designs, many of which survived until the end of the town gas era.

One such survival was Truro's and Peter Denny used it as a basis for his model at Buckingham, which I had the great privilege of seeing on many occasions. The large building on my model is, in turn, based on his.

Coal is delivered by rail at A and taken into the retort house B on a side-tipping wagon. Heat from the boiler house K carbonises the coal and the resultant gas is fed through the exhauster house C to the condenser D, where the gas is cooled. Large particulates suspended in the gas are removed in the scrubber F, then ammonia is removed in the washer G. The final stage of refining is the purifier H - three tanks, containing iron oxide. Passing the gas though these removes sulphur – the culprit for the rotten egg smell that always seemed to pervade these old gas works. Finally, the now fully refined gas passes through the meter house I, to be stored in the gas holder J.

After gas is removed from the coal it becomes coke. This is removed from the retort house on a second side-tipping wagon and deposited at L for resale. One might expect this to be removed by rail, but at St Petrock the coal yard of Messrs Giles and Powell is nearby, and it all goes there (or rather, it will do when I provide them with a suitable lorry). E is a tank that collects tar from the retort house, condenser and scrubber. M, sited at the works entrance, is the office.

The wagon is standing on dual-gauge track at the coal delivery point. The narrow-gauge side-tipper at the back would be hand-pushed into the retort house. Coke comes out of the retort house on the line nearest the camera.

For details about the next three items I am indebted to Countryside Models for photos of their exquisite model gas works – far nicer than mine. 

The tall structure behind the tar tank is the condenser.  Quoting from Countryside Models...

"The condenser itself is a piece of equipment which helps date the model.  So far modellers have reproduced either early air-cooled serpentine condensers or the Victorian tubular "Annular Condensers" but these were replaced from the 1930s onwards by more efficient condensers in which electricity was used to pump water through the gas in tubes (like a loco boiler in reverse)."

I really must do something about the empty space behind the condenser and the rather obvious join in the backscene.  My gasworks doesn't yet have a workshop for the maintenance fitters, so this is where it will go.

The scrubber and washer, with the tar tank to the right. I presume that the building beneath the tank contained some means of keeping the tar warm and liquid.

The oxide tanks.  Poo.. what a smell!

The gas holder started life as a Hornby Scaledale model. I believe it was based on one at Fakenham but, to my mind, was much too small.  I therefore sliced it in half, added a tall centre section and built my own steelwork around it, using Evergreen styrene mouldings.

The photo on the right shows the gas works early in its construction, with the unmodified Hornby gas holder. I hope you'll agree that mine is an improvement.

Trackwork is complete

Last week I laid the final piece of trackwork. There may well be minor changes in the future but the basic track plan is now complete. This is how it was originally conceived, back in November 2013...

… and this is how it's turned out in April 2019.

The most obvious change is the new revolving fiddle yard. The original one proved unworkable – just try imagining yourself attempting to turn around a 3ft 6in long cassette, loaded with a loco and four coaches, then lifting it over several other cassettes, and you'll get the idea! By comparison, the new arrangement has been a great success and is a joy to operate.

Under construction. The original, torturous fiddle yard was on the right

Operational. The little sliding bolt aligns the tracks and completes the electrical circuit.

The baseboard that once held the cassette fiddle yard is now home to Bourdon Halt, which is based on Dunmere Halt on the former line to Bodmin North. I had always associated 'pagoda' waiting shelters with the GWR but here's proof that the LSWR had them too.

Dunmere Halt (photo: HC Casserley)

But why "Bourdon Halt"?  Well, several years ago the old pipe organ in St Illogan Parish Church was being demolished and I pocketed one of the organ stops. It was the odd one out as it was a dummy - Illogan's organ never had a 16ft Bourdon pipe. I therefore resolved that its later years would be more useful than its former ones and used it to control the turnout to Bourdon Mill.

And here is Bourdon Mill, the premises of Josiah Penrose & Sons, seed merchants and suppliers of agricultural miscellanea. Since there is no run-round at Bourdon Mill, the siding is only serviced by the morning goods train from Tregarrick North to St Petrock. Anything destined for further afield has to reverse at St Petrock and leave by one of the 'up' goods trains. 

These photos emphasises all too clearly that, though the St Petrock trackwork is complete, there remains much to do on the scenic side. St Petrock is far from being finished... perish the thought!