Marbelup Valley Railway - Signalling

In July 2003, the MVR management decided to install colour light (searchlight) signals to control train operations on the single-track main line. The initial trial installation covered the section of track between Yelverton and Freycinet, using home made electronic circuits to control the signals, and light detectors to sense train movements.

The next phase involved the installation of track circuits (track occupancy detectors), again on the same part of the layout. Installation of track circuits required considerable rewiring, as all track secions had intially been bussed together as part of the command control system.

Related to the signal installation was a plan to install a Centralised Traffic Control (CTC) system with a large panel which the Train Controller could use to oversee and control the entire railway. After evaluating options for building a hardwired control panel vs. a computer based panel, the MVR management selected a computer based system using software and electronic interface modules made by CTI Electronics in Baltimore, USA.

Deliveries of CTI equipment commenced in mid-September 2003 and by early November, track circuits, point detection and point controls were completed for the entire layout. By the end of December 2003, all signals had been installed, although at that stage, there were still 6 early prototypes to be replaced by more detailed signals. During March 2004, a further batch of etched parts allowed construction and installation of the remaining 6 signals.

Searchlight Signal

A typical searchlight signal, installed at Kojonup.

The main signal light is a 3 mm red/green bipolar light emiting diode (LED), which can also display a yellow aspect by rapidly switching between red and green.

The MVR workshops fabricated these signals from brass tube and etched brass parts. Etched parts include the signal discs, ladders and bases.

Altogether, there are 16 single-head signals similar to this one.

Bracket Signal

A bracket signal typically controls the approach to to a diverging route, as in this example at Kojonup. The upper head applies to the main route while the lower head applies to the subsidiary route which, in this case, diverges to the right.

There are 7 bracket signals on the layout.

Note for the purists: The real Kojonup station never had signals, as it was located on a lightly-used branch line. When it was incorporated into the Marbelup Valley Railway, the model of Kojonup was "upgraded" to a main line station, although it still retains the brown "pea gravel" ballast typical of lightly-built lines. The MVR management decided to install signals at Kojonup to allow full CTC control of the MVR main line.

Dwarf Signals

Dwarf signals typically control access from sidings, etc. onto the main lines. These signals are part of a group of four which control trains exiting the staging yard at Springdale North.

There are 8 dwarf signals in total.

Cantilever Bracket Signal

This cantilever signal controls the main line (at right) leaving Kojonup. There was no room to locate the signal to the right of the main line, due to the location of the gangers' sheds (for track maintenance crews), nor was there room to put the signal between the tracks.

The unusual cantilever design was inspired by a photo of a similar signal which existed at Armadale, about 25 km south-east of Perth. The Armadale signal used a timber cantilever arm from a former semaphore signal, although the MVR version represents a steel post.

The dwarf signal near the base of the main signal post controls departures from the passing loop (centre track).

Gantry Signals

In December 2004, a signal gantry was installed at Springdale North to control the two main tracks exiting the staging yard towards Springdale. The gantry replaced two dwarf signals, although two dwarf signals remain for the dead-end tracks.

The gantry is made from the well-known Dapol (Airfix) kit, with scratchbuilt signal heads.

Yelverton and Karri Junction Area

This photo shows the "busiest" section of the railway in terms of signals. The three signals in the foreground control the southern end of the passing loop at Yelverton.

In the background, is the bracket signal which controls Karri junction. The upper signal head applies to the main line which swings around to the right. The lower signal head controls the timber branch line leading to Ten Mile Gully. The branch line itself is not signalled and is "dark" territory as far as the Train Controller is concerned.

In the distance, just visible above the trees, are two more single-head signals which control northbound movements through the junction for the main line and the timber branch.

Intermediate Signals

There are back-to-back pairs of intermediate signals at the midpoints of the single line sections between Springdale and Yelverton and between Kojonup and Freycinet (pictured)

The intermediate signals are automatic and the Train Controller has no control over there state.

Shunt Signals

Most signals at stations include a shunt light, which is the small yellow light below the main signal light. When lit, the shunt light allows a locomotive to proceed past a red signal at low speed for shunting movements. On searchlight signals, the shunt light is a tiny, surface mount yellow LED.

Dwarf signals do not have a separate shunt light. Instead, the main signal light flashes yellow to indicate the shunt aspect.

The Train Controller can select "shunt mode" for a station which enables the shunt signals. When in shunt mode, the train crew can carry out their shunting without further intervention from the Train Controller. Selecting shunt mode also sets the intermediate signals either side of the station to red, to halt trains on the main line while shunting is in progress, as most shunting at the smaller stations obstructs the main line. Shunt mode also sets the departure signals at the station to red, so the Train Controller must deselect shunt mode when shunting is completed before the train can proceed to the next station.

Signal Numbering

Signals are numbered, using decals with a stencil typeface, corresponding to the numbers on the CTC panel.

Drivers find the numbers useful, for example, when contacting Train Control if stopped at a red signal.

CTI Signalman Module

This is one of the electronic modules which MVR purchased from CTI Electronics in Baltimore.

This Signalman module has 16 outputs and can control 8 LED signal heads. Each LED signal requires 2 outputs to reverse the LED polarity to obtain the red and green aspects.

The Signalman module generates the yellow aspect for LED signals by rapidly switching between red and green. From the CTI software, you can adjust the proportions of red and green to optimise the yellow shade.

Individual Signalman outputs can also be used as general purpose outputs. For example, the MVR system uses Signalman outputs to control points and to drive a wall-mounted (analogue) fast clock.

CTI Sentry Module

The CTI Sentry module has 16 inputs. The MVR uses one Sentry module at each station to input point status and track occupancy status.

The MVR Engineering Dept has installed opto-isolators on inputs to the Sentry modules. The opto-isolators provide a convenient method of interfacing higher voltages used elsewhere on the layout (20 V for track occupancy, 24 V for point status) and also protect the CTI circuits from damage from the higher voltages.

The various CTI modules communicate with the CTC computer via standard 4-wire telephone cables. The phone cables form a complete ring around the layout, with a single serial connection to the computer.

Each CTI module requires a filtered DC power source between 9 and 15 V. On the MVR, a home-made voltage regulator (7812 IC) at each station generates 12 V from the 24 V supply already available for existing relay-based point controls.

CTI make a range of other modules, some of which are specifically designed to directly control points, e.g. using twin coil or slow-motion point motors. However, MVR elected not to use these, as there were already 24 V relay logic panels at each station for point controls, and we wanted to retain the existing local point control panels.

Track Occupancy Detectors

Here is one of the home made track occupancy detectors. The circuitry is a variation on the venerable "Twin-T" detector circuit. The occupancy detectors detect a current flow of 1 mA or more in each "track circuit" section. For circuit details, see schematic (PDF)

The MVR workshops have equipped almost all rolling stock with axle-mounted resistors (20 Kohms per axle, 2 axles per wagon) to ensure that individual wagons are detected by the track circuits.


Enlarged View

Computer CTC Panel

Here is the MVR computer-based CTC panel. The CTI software provides graphic tools to design the layout schematic, based on a square grid system.

Portions of the CTC Panel can be animated to indicate the actual status of layout equipment. The point symbols indicate the actual point position. Track sections change from blue to red when that section is occupied by a train. A green track section indicates a route set ahead of a train. Points are coloured yellow when the points are locked by the interlocking logic. Gray tracks are sidings or the branch line which do not have train detection, i.e. "dark" territory. (All colours for tracks, etc. are user selectable.)

Clicking on a point or signal will change the status of the respective device, provided the requested change is not in conflict with the interlocking logic. If the requested change is not allowed, the software will display a message at the bottom the screen explaining why.

The CTI system includes its own programming language TCL (= Train Control Language) which allows implementation of virtually any required logic, e.g. for interlocking signals with points and track occupancy status. The logic for the entire MVR signaling system includes about 6000 lines of TCL code. This is not as daunting as it first sounds, as most is repetition of standard blocks of code, with minor adjustments as required for different situations.

Approach Locking

A recent change (April 2004) to the code applies approach locking to main line points. Approach locking means that main line points are locked when any of the signals controlling access over the points are clear (i.e. green or yellow) and the corresponding approach track is occupied.

The screenshot shows an example where Points 801 are locked because Signal 50 is green and there is a train on Track FRE4T.

If the Train Controller clicks on one of the points to attempt to change the locked points, a red padlock symbol pops up for a few seconds (lower photo) to indicate that the points can't be operated, and a help message is displayed at the bottom of the screen.

There is a 20 second time delay to maintain the approach locking if the Train Controller places a signal at red. In real life, this time delay (typically 90 seconds or longer) prevents the points being changed just in front of a moving train by allowing time for the driver to stop at the red signal before the point lock is released. In the event that the train cannot stop in time and runs past the red signal, the points remain locked once the track circuit through the points becomes occupied.

In model form, the approach locking is less critical but it does add operating interest and requires the Train Controller to plan ahead to avoid delays.

Another recent change disables the local point control panels unless the Train Controller authorises local "shunt" mode at that station.

MVR Home Guided Tour Construction Photos Control Systems
Updated 7 June 2005