Layout Automation — Worked Examples

4. Worked example 1 — Mega Loop: a simple two-platform passing loop

4.1 Summary

4.2 Track plan

The numbered circles are block detector vPorts (1–4). A single-track main line enters from the west, opens through the West points into a two-track loop, and recombines through the East points to continue east. Each platform has a 4–6 cm-long ABC brake section in the middle of its block.

4.3 Hardware tally

Four System 2 nodes share the CAN bus. Every part is off-the-shelf — no custom wiring, no operator panel.

4.4 vPort allocation

Block detectors get the low single-digit vPorts (matching the numbered circles on the track plan). Points and brake relays use the same numbering convention as elsewhere in this document (200s for points, 400s for brakes, 500s for internal interlock flags).

4.5 Routes

Eight routes run the entire station. Every route is fired by a block-detector edge — there are no panel buttons, no operator inputs, no manual overrides. Two routes throw the entry points on arrival, two routes throw the exit points and release both brakes when the interlock is satisfied, two routes re-engage each brake after departure, and two routes clear the departure interlock once both trains are clear of the loop.

Route 3 (and its mirror Route 4) is where the interlock lives: the route is triggered when a train arrives at one platform, but its condition checks that the other platform is also occupied. If the other platform is empty, the route is deferred — it sits in the queue and re-evaluates every time the bus state changes. When the second train arrives the condition flips to TRUE and the depart sequence fires.

There is a second, subtler interlock at departure. The two approach detectors (vP 1 and vP 3) are shared: a train heading out of the loop crosses the very same block a train heading in would. A bare occupancy detector reports presence, not direction, so without a guard the departing trains would re-trigger the approach routes (R1/R2) and throw the entry points — potentially under the train still crossing them. The fix is an internal flag, vP 500 (“loop busy”): the depart routes set it as their first slot, R1/R2 carry a condition that lets them fire only while it is not set (on fail: drop), and two falling-edge routes (R7/R8) clear it once both trains have left the approach blocks. It is the same direction-memory technique used by the bi-directional platform in the next example.

One wrinkle worth calling out: an internal flag reads UNKNOWN until something first writes it, and — unlike a block detector — a hardware-less vPort can't be “seeded” by briefly occupying a block (see §9.3). A plain is CLEAR condition fails on UNKNOWN, which would drop R1/R2 on the very first arrival after every power-on. So R1/R2 test vP 500 is CLEAR OR is UNKNOWN (a two-condition OR): only a known-SET flag — a departure genuinely under way — suppresses them, and the station comes up working from a cold boot.

Here is Route 3 (the “EB train arrives last” case) shown in the router's web editor:

The dry-run preview shows the route would defer right now because Platform 2 is still empty in the live bus state. The route sits in the deferred queue (capacity 8) and re-evaluates whenever any vPort changes — the moment the west-bound train rolls into Platform 2, the condition flips to SET and the slots fire.

All eight routes — compact summary

Each route has 1–5 slots and (where applicable) one or two conditions. There are no chains, no schedules, no feedback or RFID vPorts — the bus state is the only input.

4.6 Sequence walkthrough

A complete cycle from idle, through both trains arriving in opposite orders, the simultaneous departure, and back to idle. Initial state: both platforms empty, both brakes engaged (vP 400 = 0, vP 401 = 0), points wherever they were last left.

4.7 Variants and tuning notes

• Dwell timer for “passenger boarding”. On R3/R4, bump the delay on the second points slot (201 = 0) from 1500 ms to 10000 ms. Once both points have settled, the trains sit for 10 seconds with brakes still engaged before the releases fire — a satisfying realistic pause for the viewer.
• Stagger the departures. On R3/R4, set the Platform 1 brake slot (400 = 1) delay to 2000 ms: Platform 1's brake drops, then 2 s later Platform 2's. EB rolls out first, WB follows. Useful if your locos accelerate differently and you don't want them meeting mid-loop.
• Add a starter signal at each end. Drop two more servo channels for semaphore arms on vP 300 (East-facing, for EB departures) and vP 301 (West-facing, for WB departures). Insert a "clear signal" slot before each brake release in R3/R4. Adds visible drama without changing the interlock logic.
• Schedule the station closed at night. On R3 and R4, enable Schedule with start = 06:00, end = 23:00. Outside the window, the depart routes are dropped (status log: Route N blocked by schedule). Trains arriving overnight wait in their platforms until 06:00 and depart at first light.
• DCC instead of ABC brakes. If you're running DCC, swap the S2-Relay8 for an S2-DCC node and replace the brake-relay slots with "send speed-0 to loco at address N" commands. The interlock condition logic is unchanged.
• Longer wait windows. The 60 s defer TTL is the firmware maximum. For operations where the second train might be much further off, leave the TTL at 60 s and rely on the second train's own arrival route (R4 or R3) to fire the depart sequence — the deferred timeout doesn't break anything because the mirror route will pick up the duty.
• Three platforms or more. Add another platform vPort and extend the condition: change R3 to vP 4 is SET AND vP 6 is SET. Up to four conditions per route gives you a four-platform interlock without any other changes.
• Recovering a stranded interlock. R7/R8 clear vP 500 only when both trains have left the approach blocks. As with any edge-triggered logic, a lost CAN frame on a busy bus could leave the flag set and lock out new arrivals (the status log shows R1/R2 dropping on the vP 500 condition). Recovery is automatic on a power cycle — vP 500 resets to UNKNOWN, which the cold-start guard treats as clear — or, to avoid the reboot, add a panel “reset” button that writes vP 500 = 0 unconditionally, exactly like the bi-directional platform's all-clear button (§5.8). Do not use a timed or platform-clear auto-reset here: the platforms clear mid-departure, before the leaving trains cross their approach blocks, so any such reset risks dropping the guard while a train is still on the points.

5. Worked example 2 — Bi-Directional Platform: a three-platform station with a shared middle road

5.1 Summary & introduction

Why have a bi-directional platform?

A bi-directional middle platform doubles the station's effective capacity without doubling its footprint. Common reasons to design one in:

• Heritage and branch-line stations where peak-time charter or relief services need somewhere to terminate without occupying the running platforms.
• Loco-release moves at terminus or pseudo-terminus stations: the train arrives in P2, the loco runs round through the throat, and the train leaves in the opposite direction.
• Single-line working during disruption: when one direction of the main line is blocked, the middle platform lets trains share a single working road sequentially.
• Operational variety on a model railway — P2 lets you mix terminating, originating and overflow movements through the same station scenically.

How the Route Processor helps

Bi-directional working is the classic case where manual interlocking gets you into trouble. The router solves three problems for you:

• Direction memory. The router holds two internal flag vPorts (here vP 500 = “P2 is EB” and vP 501 = “P2 is WB”). They are written by the arrival routes and cleared on departure, giving you a stateful record of which way the platform is currently “facing” without any extra hardware.
• Conflict refusal. Each P2 approach route carries a condition that the opposite-direction flag is CLEAR. Press the wrong button and the route is dropped to the status log with a clear diagnostic — nothing moves, no points throw, no brake drops.
• Single-button ceremony. One panel button per “send to platform N” or “depart platform N” issues the whole sequence — points settle, brake releases (or engages), flags update — in the right order with the right delays. The operator never has to remember the order.

5.2 Track plan

The single-track main line enters from each end and fans out through a two-point ladder (one S2-serv8 channel per point). Each platform has its own block detector and an ABC brake section centred on the platform. Direction arrows show single-direction running on P1 (east-bound only) and P3 (west-bound only); the middle Platform 2 is bi-directional.

5.3 Hardware tally

One Route Processor, one Mini Panel (block detection plus four operator buttons), one S2-serv8 driving all four point motors, and one S2-Relay8 driving the three ABC brake sections. Eight CAN nodes from the System 2 family share the bus; nothing more is needed.

5.4 vPort allocation

Block detectors keep the single-digit/teens vPorts to mirror the numbered circles on the track plan. Points use the 220s, brake relays the 420s, and the two internal direction flags sit in the 500s. Operator buttons follow the document's convention in the 3000s.

5.5 The direction interlock

The interlock that makes the shared platform safe lives on the two P2 approach routes (R2 and R4) and the two P2 departure routes (R11 and R13). Here is Route 2 (“Send EB to P2”) shown in the router's web editor — the route that throws the West throat for an east-bound train heading into the middle platform.

Three slots, two conditions, one button. Slot 1 sets W-A through (so the train rolls past it without being diverted to P1). Slot 2 sets W-B straight (so the train continues into P2 rather than diverging down to P3). Slot 3 writes vP 500 = 1 — the “P2 is EB” flag — with zero delay because it's the last action of the route.

The conditions are the interesting bit. Both must be true for the slots to fire:

• Condition 1: vP 12 is CLEAR — Platform 2 must currently be empty. Sending a second train into an occupied platform is always wrong.
• Condition 2: vP 501 is CLEAR — the “P2 is WB” flag must be off. If a west-bound train is already approaching P2 from the East throat, P2's flag will be SET to 1 by R4 even though vP 12 itself hasn't gone occupied yet. Without this condition, you could route an EB and a WB train into the same platform from opposite ends at the same time.

The On condition fail = Drop setting is deliberate: if the operator presses the button at the wrong moment we want the action dropped immediately with a status-log line. Defer would invite a stale press to fire seconds later when the original intent is gone. The status log gets Route 2 dropped: condition 2 (vPort 501 is CLEAR) failed — the operator sees instantly why nothing happened.

5.6 All routes — compact summary

Thirteen routes run the whole station: four reception (R1–R4), three brake re-arm (R5–R7) and six departure (R10–R13 plus two mirrored brake re-arms). Numbering is grouped by purpose with gaps so you can insert a starter-signal route per platform later without renumbering.

5.7 Sequence walkthrough

An east-bound parcels train is routed into P2 (the middle road). While it sits there a west-bound stopper arrives and is correctly diverted to P3 because P2 is locked east-bound. Both trains depart, P2 clears, and the station resets. Status-log lines shown verbatim where the router emits them.

5.8 Variants and tuning notes

• Indicator LEDs on the Mini Panel. Use the 8 spare outputs to mirror vP 500 / vP 501 / vP 11 / vP 12 / vP 13 on the operator's console. Add tiny “reception” routes triggered on flag-set that write to the output vPorts; you'll see at a glance which way the middle platform is “facing”.
• Starter signals. Drop six more servo channels for semaphore arms (one for each direction-of-departure per platform: vP 320–325). Insert a “clear signal” slot before each brake-release in R10–R13, and a defensive “signal red” slot on the falling-edge routes (R5–R7) so the signal always returns to RED after the train leaves. The interlock logic doesn't change.
• Loco-runaround at P2. Add a fifth panel button “Flip P2 direction” that toggles vP 500 and vP 501 in one route. After a train arrives east-bound into P2 and the loco runs round (manually or via a separate sequence), press the button; the platform is now “facing” west and the WB depart route (R13) will fire correctly when you dispatch.
• Schedule night-time single-line working. Disable R1 and R3 (the single-direction platforms) overnight via the Schedule field, leaving only R2 and R4 active. All traffic uses P2; P1 and P3 are dark until 06:00.
• Three platforms, three directions. If P2 is genuinely the only useful through road (the others are bay platforms), extend the convention: add vP 502 / 503 / etc. for any other shared road. The pattern scales linearly — one flag pair per shared resource.
• DCC instead of ABC brakes. Swap the S2-Relay8 for an S2-DCC node and replace each brake-relay slot with a “speed-0 to address N” / “speed-30 to address N” pair. The interlock structure is unchanged — only the slot payload changes.
• Add an “all clear” reset button. Sometimes manual moves leave the flag vPorts set with no train actually present. A panel button (e.g. vP 3099) firing a one-slot route that writes vP 500 = 0 and vP 501 = 0 lets the operator force-reset the platform without a power cycle.

6. Worked example 3 — Mega Sidings: an 8-road through fiddle yard

6.1 Summary

6.2 Track plan

The points-ladder is a binary tree: three "levels" of points get any incoming train onto one of eight roads with three servo movements. Buffer ends are on the far right. The last block on each road has an ABC brake section.

6.3 Hardware tally

6.4 vPort allocation

Each "into / out of Road N" requires three points thrown on the relevant ladder. The binary index of the road, mapped to (P1, P2L/P2R, P3a/b/c/d), is identical for the west and east sides — same bit pattern, different physical servos.

6.5 Block groups

Ten groups: one per road plus one for each mainline approach. The mainline-approach groups are dual-purpose — "is there a train waiting to come in?" (SET test) and "is the path clear for departure?" (CLEAR test).

6.6 Routes

With symmetric entry/exit there are five route classes per road:

1. Receive from west — throw the west ladder to road N, clear west home signal.
2. Receive from east — throw the east ladder to road N, clear east home signal.
3. Auto-brake on arrival at mid-block — engage brake relay when the train hits B3 (works for arrivals from either direction).
4. Depart to west — throw west ladder, clear west-facing starter, release brake.
5. Depart to east — throw east ladder, clear east-facing starter, release brake.

That's 5 × 8 = 40 routes for the basic yard, well under the 100-route limit. We'll detail the Road 1 set; copy the pattern for roads 2–8 with the substitution table below.

Route 1 — "Receive from west into Road 1"

Operator presses the "Receive from west into Road 1" panel button (vP 3010). Conditions check that Road 1 is empty and the west approach actually has a train waiting. Slots throw the three west-ladder points to route into Road 1, then clear the west home signal.

Route 2 — "Receive from east into Road 1"

Mirror of R1, but for an eastbound train arriving at the east throat. Uses the east-ladder points (P1R, P2Ra, P3Ra) and the east home signal.

Route 3 — "Train at B3, engage brake"

Auto-fires when the middle block of Road 1 (B3 = vP 1103) goes SET. The brake snaps on, stopping the train mid-road. Direction-agnostic: works whether the train arrived from the west or the east, because B3 is detected the same way either way.

Both home signals are stamped back to RED — the throat is no longer "cleared" for this incoming train, so neither side accidentally accepts another arrival down the same road.

Route 4 — "Depart from Road 1 to the west"

Operator presses "Depart Road 1 westbound" (vP 3030). Conditions: a train must be present on Road 1 (group SET) and the west approach must be clear (no incoming train using the throat). Slots throw the west ladder back out to Road 1, clear the west-facing starter signal, release the brake.

Route 5 — "Depart from Road 1 to the east"

Mirror of R4 for an eastbound departure. Uses the east ladder and the east-facing starter signal.

Replicating for Roads 2–8

Same five routes per road (40 routes total for the basic yard), with these substitutions for road N:

6.7 Brake control — optional "sail-through" override

By default the brake engages the moment a train hits B3, stopping it dead-centre of the road. That's what most operators want — the yard parks the train cleanly for storage. But with the symmetric layout it's natural to also use the yard as a through-running route: a non-stop train enters from one side, sails through a chosen road, exits the other side, never stops.

For that we use the same "Disable brake on next departure" pattern, applied at the moment the train reaches B3 rather than at departure time. Set the panel toggle vP 3050 BEFORE the train arrives; the train enters B3, the auto-brake route (R3) is shadowed by an override route (R6) that fires concurrently:

6.8 Sequence walkthrough

An eastbound train approaches the west throat. Operator picks Road 4 (it's empty). After a session break, the train is dispatched onwards to the east. Status-log lines shown verbatim.

Note the symmetry: this same train could equally have been dispatched back to the west (R34 instead of R35), in which case it would have used the WEST ladder + west-facing starter. Roads in a through yard genuinely behave as bidirectional sidings.

Sail-through scenario (override active)

Operator pre-sets vP 3050 (the brake-disable override) and the east-facing starter for Road 4 BEFORE the train arrives. The arrival sequence becomes:

• t=0–9 as above.
• t≈15.5: B3 fires. R33 engages brake. R36 (override) ALSO fires — vP 3050 is SET, so the brake is immediately released and the override consumed.
• The loco coasts through B3 without stopping; the east-facing starter is green; the train exits eastbound at line speed.

7. Worked example 4 — Automated Fiddle Yard Exit (Multiple points)

7.1 Summary

7.2 Track plan

Each of the four roads carries a sense block in the same vertical band — the detection zone. The two ladder points fan in to short trunk sections; a third point merges those trunks onto the single exit road. Trains travel left → right.

7.3 Hardware tally

Still only three System 2 nodes — the same parts as the simple example, just using more channels.

7.4 vPort allocation

Four detection inputs in the low 10s, three point servos on the 200 series.

7.5 Routes

Four routes, two slots each. Each route writes both of the points on the exit path for its road. The two slots fire in immediate succession — the engine staggers consecutive slots by at least 50 ms (this route sets an explicit 100 ms gap). Because the two points are driven by different physical channels, that fraction-of-a-second stagger is imperceptible, so the servos appear to swing together.

7.6 Sequence walkthrough

A typical exit from R3 with the ladder set the wrong way (previous train came out of R1):

1. t=0 s. Train sits stationary on R3. All sense vPorts clear. Points are: vP 200 = 0 (upper ladder to R1), vP 201 = anything, vP 202 = 0 (exit to upper trunk).
2. t=1 s. Operator opens the throttle. The R3 loco creeps forward toward the lower ladder point.
3. t=4 s. Loco enters the R3 sense block. The Mini Panel writes vP 12 ← 1.
4. t=4.01 s. R3's route triggers on the rising edge. Slot 1 writes vP 201 ← 0 — the lower ladder begins swinging to R3.
5. t=4.11 s. Slot 2 fires (after the 100 ms inter-slot delay). Writes vP 202 ← 1 — the exit point begins swinging to the lower trunk.
6. t≈5.5 s. Both servos finish. The status log shows two slot writes for route R3; the exit path from R3 through both points onto the exit road is now correctly set.
7. t≈7 s. Loco reaches the lower ladder point, rolls through, continues onto the lower trunk.
8. t≈8 s. Loco reaches the exit point, rolls through onto the exit road.
9. t≈10 s. Tail of the train clears the R3 sense block; vP 12 returns to 0. No route fires — the falling edge is not configured.

Note that vP 200 (the upper ladder) was never touched — it stays set to R1 from the previous operation. That's fine: no train is on the upper trunk and nothing about the R3 exit cares where the upper ladder is pointing.

7.7 Variants and tuning notes

• Adding a fifth or sixth road. If the extra roads share the same trunks, you can stretch one of the ladders into a 3-into-1 fan. The pattern stays the same: one extra sense block, one extra servo channel, one extra route. Each extra ladder stage just adds one more slot to the routes that pass through it.
• Per-road starter signals. Add a third slot to each route that clears a per-road signal from RED to GREEN once the path is set. The Mini Panel's eight outputs can drive four bi-colour LED aspects, or use an S2-Sig8 for prototypical 3-aspect signals.
• Path-clear interlock. Add a condition "exit-road sense block is CLEAR" with Defer 10 s on each route, so a train won't be released into the back of another train still occupying the exit road. This is the first interlock and is independent of which road is exiting.
• Per-road brakes (ABC). Wire a relay per road and chain each route to a "brake release" route on the same road. Now nudging the controller in any road triggers the full ceremony: detect → throw points → release the brake on that road. The operator never has to wonder which road is “armed”.
• Be careful with simultaneous activations. As in the simple example, if two trains both enter sense blocks at the same moment, both routes fire and the last write on vP 202 wins — one of the trains will run through a point set against it. The forthcoming “complex” build pairs each route with a mutual-exclusion lock to serialise concurrent exits.
• Sense-block siting. The same rule as the simple example applies: the sense block must be far enough back from the first point the train hits that the servo has time to swing. Both ladder servos and the exit servo swing in parallel, so the time budget is one servo swing (~1.5 s), not three.