In this diagram, relay 2H is normally energized, and relay 4H is normally de-energized.

Marked "F" are FRONT contacts: they are made when the relay is on, or PICKED.

Marked "B" are BACK contacts, which are made when the relay is off, or DROPPED.

"FB" contacts are dependent "FRONT-BACK" contacts, commonly known as "form-C."

The "is greater than" symbol at the left end of to bottom line symbolizes positive voltage, "BATTERY," at that point. The arrowhead points to ground, "COMMON"

The "normal," i.e. system-just-powered-up, state of the relay in question is reflected in the drawings since some vitals spend 99% of their lives powered up, their "normal" state is on, unlike most circuit conventions which draw all devices in the off position.


Generally, the selectors are "probes" that will pick up if a particular route is available over a switch.
PBS and XS pick up when an Entrance in initiated, or and exit is selected, respectively. These will be covered later, as their operation is dependent on the state of their adjacent selectors.
Each possible route over a given switch has one selector for each direction. 5 switch has:
5NE, 5 normal east
5NW, 5 normal west
5RE, 5 reverse east
5RW, 5 reverse west
A and B ends of a crossover have the same "reverse" relays, but separate normals. 9 crossover has:
9ANE, 9 A Normal East
9ANW, 9 A Normal West
9BNE, 9 B Normal East
9BNW, 9 B Normal West
9RE, 9 Reverse East
9RW, 9 Reverse West
Each selector has pickup contacts, on the right of the diagrams, and lockout contacts, on the left. Most of the lockouts are shared between two or more coils.

Considering a route from and entrance at signal 2 to an exit at signal 8:

2PBS will pickup when the entrance button is pushed, causing a "cascade" of westward selectors to pick up in sequence.
1st: 3ANW and 3RW pick for the two possible routes westward over switch 3A.
2nd: 9BNW and 5NW pick.
3rd: 9ANW picks, but 9RW does not, as it is locked out by 9BNW. Relay 9RW breaks over a back contact of both 9BNW and 3RW; 9BNW back prevents a "converging" probe at 9B. Since 9RW picks over a longer chain, relay timing will allow 9BNW, the more direct route, to be selected. Additionally, the "runaround" move over 3 and 9 reverse is specifically prevented by 3RW back in 9RW coil wire.

When the exit is selected at signal 8, 8XS picks up, beginning the "reverse wave" back across the plant.
9BNE picks up first. Since both selectors for switch 9A are picked up, the Non-Vital network now "calls" for the switch to throw normal. One of the functions of the reverse wave is to throw the switches, the other is to drop out the selectors from routes no longer being considered.
9BNE picks 3ANE, and since a 3ANE back contact appears in 3RW, the chain of relays (3RW-5NW-9ANW) for the alternate leg all drop away.

Route Initiation and Completion; PBS-XS

So what's the deal with PBS and XS? We can see that PBS and XS are both triggered by the same button on the panel, and the state of the selectors, or "probes" condition one or the other to pickup. Also note that each one has a "pickup" circuit, and a "stick" circuit which includes a contact on the relay itself.

2PB is the panel pushbutton for signal 2. If a route is being considered leading up to signal 2, then either 3RE or 3ANE will be up, which cuts out the PBS circuit and allows 2XS to pick up. Before this can happen, 2XL [ 2 eXit Lock ] is checked. 2XL picks up when all conditions down the line like opposing traffic, which would prevent a route with an exit at 2, have been checked. The next two checks are 3ANW back and 3RW back, to be sure that 2XS will not pick up if there is already an initiation westward. 3ANE and 3RE appear again on the negative side of the coil because one of these two is the only condition necessary to hold the relay on once it is up. When 2XS picks, the next thing that happens is 3RW or 3ANW energizing and opening the wire from the button. Since 2XS is up and holding through itself (dashed line) it is effectively ignoring the rest of the circuit.

With 3ANE and 3RE both down, 2PB picks 2PBS through 9RWK back. The stick leg of When 2PBS is: 2PBS front, 2CLP [ 2 Clear Lever rePeater ], then some trickiness. 2180TP and 2H are the "auto-cancel" circuit. We will see later that 2H checks that 2180TP is up, and drops when 2180TP drops --when a train accepts a signal and shunts the track circuit. 2PBS stays picked up through one of these two contacts until that INSTANT when 2180TP front has let go, but 2H back has not yet made contact. That fraction of a second is enough time to cause 2PBS to drop away.

2SW is a switch on the panel near 2PB. If flipped up (on the diagram) it rides out the auto-cancel circuit, and 2 signal is Fleeted. If 2SW is flipped down, a momentary contact, then 2CLP picks and breaks the stick leg of 2PBS. This is the manual cancel function.

Switch Levers,
Switch Indicators

The NLP and RLP relays are picked by the operator, or the selector network, to operate switches.
[ Normal Lever rePeater ]
[ Reverse Lever rePeater ]
3NLP and 3RLP mutually exclude each other by breaking over the others back contact.
3NLP has three pickup legs, selectors for both directions on the A end: 3ANE & 3ANW, both selectors on the B end: 3BNE & 3BNW, and finally an Auxiliary Key on the operators panel.
3RLP has but one pair of selectors 3RE & 3RW, and the Aux Key.

The selectors need some way of knowing if a switch is "Locked" in position, as this affects the operation of switch selectors and PBS circuits.
NWK and RWK serve this purpose.

WK's pickup when a turnout is in correspondence, (NWC/RWC), and either the LS lock relay for the switch is dropped out, locking the switch, or the switch is called for the position in question by ~LP.

PBS relays are excluded by a trailing point switch locked against them. See 9RWK in 2PBS. Similarly, both LP's and WK's lock out selector relays, like 5RLP and 5RWK in the shared tail of 5NE and 5NW.

Signal Selectors; Networks

After all switches in a route are called, and move into position, the R Signal Selector relay picks up to clear the signal associated with the entrance button initiated.

One or two R's may pick up at the same time, depending on the routes involved. Opposing signals always lock out one another, and several others, depending on switch positions.

Control networks are used to accomplish these cross locks in the most efficient and safe manner possible.

This is the single most valuable lesson in the Phillips text: "Signal networks follow the layout of the switches in the plant."
Viewing the R network brings this clearly to light.