How does interlocking know its own arrangement?

There are interlockings that can be written this way. Westrace (Westinghouse->Invensys->Siemens) is a ladder-logic system that's used quite a bit on metro systems (including sections of London Underground) but as far as I know does not exist on the Big Railway.
An "add-on" to Westrace is a templates tool - basically, you want a point end, here's a template and it'll draw the diagram and fill in the contact/relay IDs. I think you can then modify the produced data as it's a one-way process.

SSI, and thus Smartlock and Westlock which are generally compatible variations, are heavily designed for UK main line requirements. That's what it was designed for, and it does it very well (but like all processor-based systems, slower than relays). It's good enough to be used elsewhere in the world too - Belgium and Spain come to mind for starters.

The safety assessors and suchlike prefer more automation to reduce mistakes. I think it depends on the automation though, as to how likely one is to make mistakes.

This I find odd as probably 75% of US interlockings follow one of two patterns - pairs of universal crossovers, or single-to-double track! If anything could gain from automation and/or ready-made units, this would be it! Yet it appears that the designs are written over and over again. No doubt there is a lot of replication (risking copying errors), especially with stuff like Microlok boolean/ladder logic.

6.2.5. Stack Route Commands
There are many occasions when a dispatcher wants to clear a route as soon as a conflicting situation is resolved. For
example, a dispatcher wants to clear a train out of siding as soon as a train moving in the opposite position clears the
mainline block in front of the siding Without stacking, the dispatcher has to constantly monitor the situation until there
is no longer a conflict and then clear the new route.

Stacking allows the dispatcher to enter the next route to be cleared into the system. As soon as the system sees that
there is no longer a conflict, it will automatically clear the route by positioning the switches and clearing the signal.
To set up a stacked route, place the mouse pointer on the signal you want to clear next and click the right mouse
button. A pop-up menu will appear. Select the “Add Stack Command” and the signal will begin to blink in the same
fashion as the entrance-exit command. Next find the signal where you want the stacked route to end, place the mouse
pointer on the signal, and then click the left or right mouse button. If Train Dispatcher 3 can find a path from the
beginning to the ending signal it will give you a message that the stack command was added. The stacked route will also
be displayed in yellow. Only those portions of the route that are currently unoccupied will be shown in yellow. A number
will also appear at the base of signal indicating the number of routes that have been stacked for that signal.

Unlike entrance-exit commands, stack commands can only go to the next signal. In Figure 17, if the signal shown as the
entrance signal is specified as the beginning of a stacked route, there are only two signals that could be designated as
the ending signal. These are the first signals to the right of the entrance signal on the second and third tracks from the
top of the figure.

You can designate up to five stacked routes per signal. The first stacked route you enter will be the next route cleared
when the current conflict is resolved. To see a list of stacked routes place the mouse pointer on the signal and click the
right mouse button. A pop-up menu will appear which will have a “Delete Stack Command XXX (entrance signal name) –
(exit signal name)” menu item for each stacked route. To close the pop-up menu, place the mouse pointer at any
location outside the pop-up menu and click the left mouse button.

To delete a stack command, select the appropriate “Delete Stack Command” menu item.

This may or may not be relevant, but long before I began playing SimSig I used to play Train Dispatcher, an American game, where it was possible to stack up to five routes across a junction in memory, so that after Train A passed through and completed its route, a new route was automatically set for Train B (passing the opposite way) and maybe even for Train C (same direction as Train A). This may not have been realistic, but by heck it was fun! The game has not been updated since 2006, but is still available.

<snip>
Since this thread has had a lot of feedback from American forum members, I would be interested to know whether route stacking is actually used in American signalling, and how common this is. Surely interlocking software ought to be sophisticated enough to cope with this helpful feature.

If enough people consider this matter off-topic, I will be happy to create a new one for it.

All I can say is - wow! Thanks again Keith for your marvellous erudition.

Would love to hear from you guys across the pond.

In UK practice, presetting (I have heard it called 'oversetting'generally seems to be frowned on. I'm not sure why, it would certainly even out the workload in some locations.

The Oslo example does sound potentially risky in that - at least in theoretical terms - surely there's a risk of the interlocking getting into a deadlocked state, if two conflicting routes have both been stacked and are partially locked and contending for the same objects. Presumably there is some sort of deadlock avoidance built in, for example by ensuring that two routes will always contend for objects in a consistent order.

... A few years on, they had progressed to three colours: with no route set the button was illuminated red; with a route cleared it was green; when a button was selected but the route didn't clear (either because of approach releasing for a junction, because of a preceding train still in the section, or because it was presetting over a conflicting route) a yellow indication was given.

" said:

In UK practice, presetting (I have heard it called 'oversetting'generally seems to be frowned on. I'm not sure why, it would certainly even out the workload in some locations.

I believe the "official" term is anti-preselection <snip>

I've always heard "oversetting" used to refer to setting the same route over the remains of the existing one, or possibly a different route but ending in the same signal.

Presetting, again TTBOMK, that refers to a route from one main signal to the next presetting a shunt signal (or more than one) in between.

Another funny term that was used when testing Western Region interlockings is "pumping stick" This meant re-stroking a route which had returned to red following the passage of a train. (re-picking the stick control)