Replacing Signals to Danger

Approach Locking

Approach locking is another interlocking safety feature designed to prevent a route section from being released until a driver has had adequate time to respond to encountering a more restrictive aspect than anticipated. This should be avoided wherever possible- it's better to delay a train than put a signal back to red in front of a driver. Real signallers doing this without very good reason would get into serious trouble.

To give an example of what could happen without it, if the previous (main) signal a driver passed was green (in a three-colour aspect system), he expects the next signal to be green or yellow, yellow indicating that the signal beyond that is red. If he encounters yellow, he knows to slow the train down so he can stop by the time he reaches the red. However, if the next signal is neither green nor yellow but red, he has no option but to brake as quickly as possible in order not to overshoot this signal. Barring equipment failure, the usual cause of this abrupt, unexpected change of aspect (COA) from green to red is that the signalman (you) cancelled the route just ahead of him for whatever reason.

Obviously this creates a dangerous and unacceptable situation. Sudden braking at speed may cause derailment on its own. If cancelling the route ahead also involves changing points that are immediately beyond the red signal, they may not be reset completely by the time the train arrives upon them, risking derailment again. If the route ahead was cancelled to allow another train priority, there is a risk of collision.

To overcome this, approach locking was introduced and is now used worldwide. In its simplest form, a timer is placed on each controlled signal such that if a route ahead of that signal is suddenly cancelled, the signal changes to red but points ahead of it remain at their current settings until the route "times out". The timer setting depends on the maximum speed of trains in that section; faster trains take longer to stop. in SimSig the standard timeout for main lines is 120 seconds, and 30 seconds for a shunt signal where the maximum speed is only 15 mph (more details here ).

If this were all there was to approach locking, things would be simple. Setting a route happens with only a brief point-resetting delay after you click the required signals; cancelling it, however, would force an extra timer delay, accompanied by a flashing red light on your panel until it times out (the driver, however sees only a steady red light). Indeed, this is what most often happens, in real life and in some other simulators.

However, as you have no doubt noticed, routes in SimSig can be cancelled with gay abandon, provided no trains are anywhere near them. All it takes is a right-click on the signal. When you think about it, this requires the software to "look back" to see whether indeed there is a train approaching, and if not, allow the route to be cancelled immediately without a compulsory timeout. Convenient as this is (particularly for newcomers to SimSig), it is much more expensive in real life than simple approach locking to implement and maintain, so is usually reserved for busy junctions with high traffic volume, as it enables signalmen to vary routes far more rapidly than if they had to wait for each signal to timeout. This method of approach locking is termed Comprehensive Approach Locking and is the one used in Royston and mostly throughout SimSig.

As a result, you should train yourself to think ahead as far as possible before even setting a route, in case you might want to cancel it at short notice. When the traffic density is high in your area, with trains of different types moving at different speeds, an opportunity may present itself to halt one train to let another pass by, or to cancel a route so as to release the interlocking of points that would prevent another route from being set. This might be fine in model railroads, but in SimSig so many other factors must be considered that you should weigh up carefully whether it is better to cancel a route ahead of a train or simply let it proceed. Obviously, the answer to this is experience and advance planning; like many other things in life, the further you can anticipate this happening, the better able you will be to deal with it when it arises.

Hint: Pretend to yourself that comprehensive approach locking does not exist in SimSig, i.e., every signal cancelled has a timeout on it; this will force you to choose the routes you set very carefully!

Adverse Change of Aspect (ACOA)

This is a term specific to SimSig that defines the situation described above wherein a driver encounters a signal with a more restrictive aspect than anticipated, usually as the result of the route ahead unexpectedly being cancelled.

Although the scenario usually imagined is that of a red signal coming after a green, the wording of the definition allows other aspect changes, such as green to yellow in a four-colour system (bypassing the double yellow), or a yellow actually changing to red before his eyes as he approaches it at high speed! SimSig deals with any unexpected aspect change by taking two actions. The first is to impose the required timeout at the signal that has been changed and to display a message such as 120 seconds approach locking. The second is to display the message [Train ID] received an adverse change of aspect at [signal ID]. You will soon receive a telephone call from the driver (Driver of [train] reporting an adverse change of aspect at signal [number]). Even though the route may have cleared by now, you must answer this call, usually to tell him Driver, please continue after speaking to your control (the driver's Controller, not you!). This wastes additional time before he can start moving again, and the inevitable delay will cost you performance points. Not to mention delays to any other trains that cannot progress further while interlocking blocks resetting the points for their next routes. So by cancelling a route to cause an ACOA you will incur at least two extra time delays and be penalized as well. It might have been better for you to let him pass!

Note that causing a signal timeout delay does not necessarily lead to an ACOA; the two are different. Let's demonstrate this with 1C77, which enters from Shepreth at 06:23. To make it clearer we'll display track circuits, uncheck Panel signals and disable the Automatic button on signal 104. Here 1C77 enters (it does not stop at Meldreth), and a route has been set for it to Royston P1 and beyond.

:usertrack:sims:royston:roystontutor:app_lock_1c77_enters_2.png

While it is approaching 104, we cancel the route between K986 and K984. Although 1C77 has not yet entered the section 104-K986, SimSig's comprehensive approach locking detects a train in the previous section, so invokes approach locking at K986; its aspect changes to danger and begins to flash, while the message 120 seconds approach locking appears. Notice too that this has caused 104's aspect to change to yellow; however, the ACOA message does not appear. This is probably because 1C77 is beyond the 'sighting distance' of 104, the distance at which a driver is able to see the next signal. Were he any closer than this, he would see it change from yellow to green before his eyes and raise an ACOA. Since the display only indicates that a certain track circuit is occupied, you have no way of knowing exactly where the train's current location to be when you cancel the route ahead; but it serves as a reminder that any train should be at least one green signal away from one you wish to replace to danger (i.e., cancel the route from it), to ensure that no ACOA is caused.

:usertrack:sims:royston:roystontutor:app_lock_120_secs.png

Let's restart the sim where 1C77 enters, but this time wait until it has just entered the block K986-K984 before we set K986 to danger.

:usertrack:sims:royston:roystontutor:acoa1.png

This time there is no escaping an ACOA, even though 1C77 has not made it as far as the TC immediately before K986! This is because when 1C77 passed 104, it was green (see first pic in this series).

ACOA and stationary trains

Even if 1C77 happens to be stationary at Royston when you replace its starter signal (K980) to danger, you will still incur an ACOA in addition to the two minute approach locking timeout. Although this might seem unfair, it occurs because in SimSig there is no way for you to inform the driver beforehand of your intention to replace the signal. This actually follows normal operating procedures, which discourage a signalman from contacting the driver (but not the other way round!) as this might distract him at a time when his hands are full.

It might also be argued that there should be no need to invoke two-minute approach locking, either, when a train is stationary at a signal which is being changed. Remember that approach locking is meant to be a fail-safe system protecting the route ahead of the signal for not just the train behind it but also for any others in the vicinity making use of the same interlocking (e.g., other trains exiting from adjacent platforms). Moreover, interlocking cannot tell whether a train occupying a TC is stationary or moving. All it can do is prevent the route from releasing until a certain time has elapsed.

Correcting a wrong route setting

From time to time, you will set a wrong route for a train at a junction. Should you do this, the driver of the train will realise, stop at the offending signal and report the matter. Tell him to wait while you cancel the route, cancel the route, wait for the approach locking to time out and set the correct route. In this case, there will be no ACOA penalty because you told the driver what you were going to do in advance.

Should you find yourself unsure of what to do with a train, you may wish to save the simulation so you have somewhere to go back to should you get it wrong.

Approach locking and train delays

On the other hand, should a train be delayed at a platform or intermediate signal for a variety of reasons, including wrong food trolley (a standing joke in SimSig), the driver will be the first to let you know, then ask you whether you wish to replace the signal at which he is standing to danger, or leave it at its current setting. If you leave it unchanged, this will not incur any timeout delay and may seem the easiest course to you; however, other trains in the vicinity will be unable to take advantage of any part of it until the delay is over, which could take ten minutes to an hour or more! So the best policy, particularly if you are still unfamiliar with the sim, is always to replace the signal to danger. Having chosen this course of action, you should now replace the signal immediately after speaking to the driver. Again, you won't incur an ACOA penalty as you talked to the driver beforehand.

Overshooting signals

You might wonder whether it is possible for a train to overshoot a signal set against it at the last moment. This does not happen in SimSig- trains will do 125 to 0 in two seconds if need be. Signals Passed At Danger (SPADs) have caused fatal accidents in the past, and can have a psychological effect on drivers and signallers. As such, they are not appropriate for SimSig to simulate.

As an aside, if you replaced an automatic signal to danger, you might be surprised to discover that there is no approach locking timeout delay here (signals K973, K974), since it takes place on a section of track controlled by automatic signals, i.e., if you can't set a route through it, you can't cancel it, either!