Plymouth Citadel - Design Notes
I have looked at a lot of prototype track plans over the years, but found none to be suitable for such a small but dense layout. My original idea was to model one of my two favourite GWR stations: Penzance or Plymouth Millbay. Both were busy terminal stations with interesting traffic patterns, but not really practical in the available space. So, a freelance design was needed.The track plan is a heavily mutated version of Plan 9 from C. J. Freezer's "N Gauge Track Plans". I opted for a larger station with more extensive MPD facilities and carriage sidings, but with no goods facilities. The basic concept of combined out-and-back and double track continuous run was kept, but rearranged to put all the pointwork at the front of the layout and make it look like a stretch of four track main line.
The station is on an upper level with all the running track on a lower level below. No lower level track crosses over itself.
Gradients are critical for this type of layout. To investigate these, I stuck two yards of track onto a plank and then propped it up at various angles. I then tried running all of my locos up the slope with various loads. The conclusion was that all of my older Farish tender locos could handle 7-8 coaches on a straight 1 in 50 gradient. As a result, the back straight is at 1 in 50 reducing to 1 in 80 on the curves. Given the length of the layout, this gives just enough clearance as the main line ducks under the buffer-stop end of the station. Care was taken to ensure that no point motors or uncouplers would interfere at that end of the station. The orientation of the layout was also chosen so that trains climbing the hill would be on the outer line of the double track main line. This slightly reduces friction on the curves (due to the larger radius) and gives an slightly easier gradient (a 180 degree turn is about 3.5 inches longer on the outer track).
The designed minimum radius curve is 12 inches, although this drops to 11 inches at the end of one of the station headshunts. A larger radius would have been nice, but this would further increase the width of the baseboard and reduce the effective length of the station. This radius effectively determined the width of the layout.
Goal #1 - Single board portable layout
For simplicity, the layout was to be a single baseboard. Any station such as this with dense trackwork would be a nightmare if split down the centre into two boards: there would be something like 20 tracks at the board join, and this would be right in the middle of complex pointwork.
In practice, it had to be constructed as separate upper and lower boards for maintenance reasons. There is a lot of trackwork below the station and some of the station under-board equipment and wiring would be inaccessible if built as a single board. The join between these boards consists of just the two main line tracks. The track join is handled with removable pieces of settrack with sliding fishplates at one end.
When it comes to layout size, obviously the bigger the better, since this allows a more complex station and longer trains. The baseboard size was chosen to be the largest size I could manhandle alone, and which would fit into a standard (Australian) station wagon. This works out to roughly the size of a standard door. The final length was chosen to be 6' 6" - two inches shorter than a door. This allows it to be maneuvered around the house in an upright position. Two recessed castors at one end simplify this process. The width was determined by the track plan and minimum radius curve, and ended up as 2' 11''. This also permits any part of the layout to be easily reached as long as access to both sides is available.
Goal #2 - Medium size terminus and MPD
Goal #3 - Full passenger and loco operations, but no goods (other than the MPD)
The track plan of the terminus was squeezed to maximise the train length. Of the six platform faces, three can handle seven coach trains and one can take eight. Carriage sidings were put next to the platform roads instead of down the line and around the corner to avoid the need to park a shunting loco at the end of the platform (and so reduce train lengths by one coach). The carriage sidings are too short for full length trains, but the old GWR was very fond of remarshalling its passenger trains so this should not be too much of a problem.
The loco depot is based on standard GWR practice, although having loco storage roads off the turntable is unusual. Since I had a Fleischmann turntable lying around, the temptation to use this feature was too great to resist!
The MPD also permits some token goods handling: typically had one train a day bringing in coal and assorted supplies, and taking away loaded ash wagons.
Goal #4 - Both out and back and continuous run
A terminus-type layout requires either a fiddle yard or an out-and-back loop. I have never been fond of fiddle yards, and they don't adapt well to automatic or computer control (or tender engines), so out-and-back it was. The original C. J. Freezer design did a nice job of including a double track continuous run, which is also a very desirable feature for an exhibition layout and for running-in locos.
I considered putting in one or more storage loops to allow the order of returning trains to be altered, but the continuous run can perform the same role. Each oval is just long enough to be broken into three sections that can each hold a maximum-length train. Two trains can follow each other around each oval with some automation. Either train can then be passed on to either the reversing loop or the station approach, allowing some degree of overtaking. Automating this part is a bit more complicated than the simple out-and-back queue, so will be postponed for now.
Initially, the out-and-back circuit will be set up as a simple queue of up to five trains using some basic optical detectors and relay logic. Each continuous run will be considered one section in this queue, and the train will either be stationary or moving depending on the setting of the appropriate crossover at the front of the layout. In the future each oval will hold two trains increasing the total capacity to seven trains. This must be able to handle multiple unit trains, where the loco can be propelling from the rear.
The low level trackwork is set up to look like a four track main line with a single track branch (the reversing loop). In practice, the branch is always used instead of one of the main lines. The main line in question is obviously out of service, with a track gang relaying the track!
Goal #5 - intensive operation, for 1-3 operators
While the station is a freelance design of no particular location, the operating pattern and timetable will be based on the situation at Plymouth, treating the station as a combination of Millbay and North Road, simplified down to an appropriate level. Full timetable operation is planned, using a fast clock. Through trains to and from Cornwall will need to change engines and direction here. This also gives four distinct flows of traffic: London, Cornwall, through and local services.
A steam-era model terminus involves a fair amount of shunting for each train. The GWR frequently remarshalled its passenger trains, even adding or removing sections of through trains. There will also be a fair amount of mail and parcels traffic.
The layout controls will eventually consist of a full simulated lever frame for points and signals, and two general-purpose cabs/controllers. Initially, a simple diagrammatic panel is being used for point control. This might also be preserved into the future to simplify exhibition working. The main line out-and-back queue is handled by a third and fourth controller and some simple automation. This should be enough to keep three operators busy, especially if the clock speed is increased. A single operator can run the layout using a single cab.
Most of my locos and passenger stock have been converted to use B&B couplings. These are quite nice little auto couplers, very different to better known types such as KayDee or Microtrains, but with similar capabilities. The layout has a full complement of 14 electromagnetic uncouplers, allowing (in theory at least) full hands-off operation.
Goal #6 - Possible exhibition layout
The layout is intended to be exhibitable, depending on how things eventually turn out with respect to reliability and scenic quality. Most exhibition layouts down here in Australia tend to be quite stereotyped in concept: a double-track continuous run with a station at the front and storage loops at the back. They are also usually quite large, and quite impressive. It is rare to see any out-and-back or point-to-point designs, so this layout would be distinctive.
The main difficulty with a terminus-type layout at an exhibition is the amount of operator effort required to run it (the main reason for the popularity of the more traditional type). The combination of out-and-back, continuous run and automatic queuing should make the concept practical. A simplified operating pattern would be required, probably just changing engines for a quick turnaround.
Goal #7 - Initially to use simple (though unconventional) DC control systems
Goal #8 - Interlocked lever frame and (more or less) fully signalled
Goal #9 - Eventual upgrade to modest computer support (lever frame/signal box, automatic out-and-back, smart controllers)
Many modellers scratchbuild buildings, rolling stock, locomotives, even track. This is considered normal, even admirable. I like to scratchbuild control systems. Most modellers treat the control system as an afterthought. It really should be designed in parallel with the layout: each affects the usability of the other.
In the short term, I intend to show just how much can be done with simple stuff - relays and basic electronics. In the longer term, I intend to refit with some simple computer enhancements. FWIW, professionally I am an embedded software engineer, so using electronics and small computers to do all sorts of weird stuff is my bread and butter.
The interim control system will be a little-used variant of cab control that the Americans call "Engineer-type Cab Control". With a conventional cab control system, each section has a switch that selects the controller that will connect to it. Instead, each controller will have a switch that selects which section it will control. This layout will require only 5 sections: the 2 main lines and 2 headhunts in the station throat and 1 more for the MPD. From these five feeds, everything else will be routed through the pointwork. In addition, every siding in the station has a loco-length parking section at the end. The two main lines will treat the handoff sections to the out-and-back loop as the same sort of parking section. This means that each cab will be controlling a long length of single track with a parking section at each end. Each cab will thus need one 6-position rotary switch and 3 on-off switches. Each on-off switch has a track circuit and LED: green for available, yellow for loco present, red for loco parked. That's all of it - the entire control system - just 4 switches.
The eventual lever frame will be an array of toggle switches, fully interlocked in software, with signal repeaters and locking indicators for critical pointwork. The layout will not be fully signalled, but close to it: all movements should be controlled by running or ground signals, but there won't be as many as the GWR liked to use. Ground signals will be small bi-colour LEDs behind a non-moving disc. The running signals might be colour light, the new Dapol semaphores, or I might try my hand at home-built semaphores - not sure yet.
Goal #10 - fully functional with reasonable scenery within 6 months
A traditional rule of thumb in software engineering is to multiply all time estimates by pi, so 6 months * 3.14 is ...
The station is on an upper level with all the running track on a lower level below. No lower level track crosses over itself.
Gradients are critical for this type of layout. To investigate these, I stuck two yards of track onto a plank and then propped it up at various angles. I then tried running all of my locos up the slope with various loads. The conclusion was that all of my older Farish tender locos could handle 7-8 coaches on a straight 1 in 50 gradient. As a result, the back straight is at 1 in 50 reducing to 1 in 80 on the curves. Given the length of the layout, this gives just enough clearance as the main line ducks under the buffer-stop end of the station. Care was taken to ensure that no point motors or uncouplers would interfere at that end of the station. The orientation of the layout was also chosen so that trains climbing the hill would be on the outer line of the double track main line. This slightly reduces friction on the curves (due to the larger radius) and gives an slightly easier gradient (a 180 degree turn is about 3.5 inches longer on the outer track).
The designed minimum radius curve is 12 inches, although this drops to 11 inches at the end of one of the station headshunts. A larger radius would have been nice, but this would further increase the width of the baseboard and reduce the effective length of the station. This radius effectively determined the width of the layout.
Goal #1 - Single board portable layout
For simplicity, the layout was to be a single baseboard. Any station such as this with dense trackwork would be a nightmare if split down the centre into two boards: there would be something like 20 tracks at the board join, and this would be right in the middle of complex pointwork.
In practice, it had to be constructed as separate upper and lower boards for maintenance reasons. There is a lot of trackwork below the station and some of the station under-board equipment and wiring would be inaccessible if built as a single board. The join between these boards consists of just the two main line tracks. The track join is handled with removable pieces of settrack with sliding fishplates at one end.
When it comes to layout size, obviously the bigger the better, since this allows a more complex station and longer trains. The baseboard size was chosen to be the largest size I could manhandle alone, and which would fit into a standard (Australian) station wagon. This works out to roughly the size of a standard door. The final length was chosen to be 6' 6" - two inches shorter than a door. This allows it to be maneuvered around the house in an upright position. Two recessed castors at one end simplify this process. The width was determined by the track plan and minimum radius curve, and ended up as 2' 11''. This also permits any part of the layout to be easily reached as long as access to both sides is available.
Goal #2 - Medium size terminus and MPD
Goal #3 - Full passenger and loco operations, but no goods (other than the MPD)
The track plan of the terminus was squeezed to maximise the train length. Of the six platform faces, three can handle seven coach trains and one can take eight. Carriage sidings were put next to the platform roads instead of down the line and around the corner to avoid the need to park a shunting loco at the end of the platform (and so reduce train lengths by one coach). The carriage sidings are too short for full length trains, but the old GWR was very fond of remarshalling its passenger trains so this should not be too much of a problem.
The loco depot is based on standard GWR practice, although having loco storage roads off the turntable is unusual. Since I had a Fleischmann turntable lying around, the temptation to use this feature was too great to resist!
The MPD also permits some token goods handling: typically had one train a day bringing in coal and assorted supplies, and taking away loaded ash wagons.
Goal #4 - Both out and back and continuous run
A terminus-type layout requires either a fiddle yard or an out-and-back loop. I have never been fond of fiddle yards, and they don't adapt well to automatic or computer control (or tender engines), so out-and-back it was. The original C. J. Freezer design did a nice job of including a double track continuous run, which is also a very desirable feature for an exhibition layout and for running-in locos.
I considered putting in one or more storage loops to allow the order of returning trains to be altered, but the continuous run can perform the same role. Each oval is just long enough to be broken into three sections that can each hold a maximum-length train. Two trains can follow each other around each oval with some automation. Either train can then be passed on to either the reversing loop or the station approach, allowing some degree of overtaking. Automating this part is a bit more complicated than the simple out-and-back queue, so will be postponed for now.
Initially, the out-and-back circuit will be set up as a simple queue of up to five trains using some basic optical detectors and relay logic. Each continuous run will be considered one section in this queue, and the train will either be stationary or moving depending on the setting of the appropriate crossover at the front of the layout. In the future each oval will hold two trains increasing the total capacity to seven trains. This must be able to handle multiple unit trains, where the loco can be propelling from the rear.
The low level trackwork is set up to look like a four track main line with a single track branch (the reversing loop). In practice, the branch is always used instead of one of the main lines. The main line in question is obviously out of service, with a track gang relaying the track!
Goal #5 - intensive operation, for 1-3 operators
While the station is a freelance design of no particular location, the operating pattern and timetable will be based on the situation at Plymouth, treating the station as a combination of Millbay and North Road, simplified down to an appropriate level. Full timetable operation is planned, using a fast clock. Through trains to and from Cornwall will need to change engines and direction here. This also gives four distinct flows of traffic: London, Cornwall, through and local services.
A steam-era model terminus involves a fair amount of shunting for each train. The GWR frequently remarshalled its passenger trains, even adding or removing sections of through trains. There will also be a fair amount of mail and parcels traffic.
The layout controls will eventually consist of a full simulated lever frame for points and signals, and two general-purpose cabs/controllers. Initially, a simple diagrammatic panel is being used for point control. This might also be preserved into the future to simplify exhibition working. The main line out-and-back queue is handled by a third and fourth controller and some simple automation. This should be enough to keep three operators busy, especially if the clock speed is increased. A single operator can run the layout using a single cab.
Most of my locos and passenger stock have been converted to use B&B couplings. These are quite nice little auto couplers, very different to better known types such as KayDee or Microtrains, but with similar capabilities. The layout has a full complement of 14 electromagnetic uncouplers, allowing (in theory at least) full hands-off operation.
Goal #6 - Possible exhibition layout
The layout is intended to be exhibitable, depending on how things eventually turn out with respect to reliability and scenic quality. Most exhibition layouts down here in Australia tend to be quite stereotyped in concept: a double-track continuous run with a station at the front and storage loops at the back. They are also usually quite large, and quite impressive. It is rare to see any out-and-back or point-to-point designs, so this layout would be distinctive.
The main difficulty with a terminus-type layout at an exhibition is the amount of operator effort required to run it (the main reason for the popularity of the more traditional type). The combination of out-and-back, continuous run and automatic queuing should make the concept practical. A simplified operating pattern would be required, probably just changing engines for a quick turnaround.
Goal #7 - Initially to use simple (though unconventional) DC control systems
Goal #8 - Interlocked lever frame and (more or less) fully signalled
Goal #9 - Eventual upgrade to modest computer support (lever frame/signal box, automatic out-and-back, smart controllers)
Many modellers scratchbuild buildings, rolling stock, locomotives, even track. This is considered normal, even admirable. I like to scratchbuild control systems. Most modellers treat the control system as an afterthought. It really should be designed in parallel with the layout: each affects the usability of the other.
In the short term, I intend to show just how much can be done with simple stuff - relays and basic electronics. In the longer term, I intend to refit with some simple computer enhancements. FWIW, professionally I am an embedded software engineer, so using electronics and small computers to do all sorts of weird stuff is my bread and butter.
The interim control system will be a little-used variant of cab control that the Americans call "Engineer-type Cab Control". With a conventional cab control system, each section has a switch that selects the controller that will connect to it. Instead, each controller will have a switch that selects which section it will control. This layout will require only 5 sections: the 2 main lines and 2 headhunts in the station throat and 1 more for the MPD. From these five feeds, everything else will be routed through the pointwork. In addition, every siding in the station has a loco-length parking section at the end. The two main lines will treat the handoff sections to the out-and-back loop as the same sort of parking section. This means that each cab will be controlling a long length of single track with a parking section at each end. Each cab will thus need one 6-position rotary switch and 3 on-off switches. Each on-off switch has a track circuit and LED: green for available, yellow for loco present, red for loco parked. That's all of it - the entire control system - just 4 switches.
The eventual lever frame will be an array of toggle switches, fully interlocked in software, with signal repeaters and locking indicators for critical pointwork. The layout will not be fully signalled, but close to it: all movements should be controlled by running or ground signals, but there won't be as many as the GWR liked to use. Ground signals will be small bi-colour LEDs behind a non-moving disc. The running signals might be colour light, the new Dapol semaphores, or I might try my hand at home-built semaphores - not sure yet.
Goal #10 - fully functional with reasonable scenery within 6 months
A traditional rule of thumb in software engineering is to multiply all time estimates by pi, so 6 months * 3.14 is ...