Modelling Time Locks on Switches

Time Locks – An Introduction

In the real world, manual switches within signalled territory are protected by devices called “time locks”.  The purpose of these is to prevent a switch from being opened in the face of an approaching train.  When the conductor wants to open the switch, he unlocks it and starts the timer running (how this is done depends on the model of time lock).  The time delay gives any train too close to stop – or sometimes too close to even see a restricting signal – time to safely pass over the switch before the points are changed.  It also triggers the signal system to display restricting aspects around the block, so trains that are further out are alerted to the presence of an open switch.

Once a programmed amount of time has passed, the timer indicates to the user that it has expired (often by a white or green light) and then releases a locking mechanism that allows the points to be moved manually.   (This is commonly done with a locking pin through the throwbar that is retracted, but there are other mechanisms.)

Time locks aren’t just a good idea – they’re required by law here in the US.  Under 49 CFR 236.207, either approach or time locking is required of manual switches in signalled territory.

How Time Locks Prevent Accidents

Understanding time locks – it’s not that hard!

I promise -understanding time locks isn’t as hard as these six guys make it look!  They’re trying to operate what appears to be a GRS/Alstom Model 10 machine, which is the black box in front of the guy’s legs.   (To be fair, they’re not confused – the time lock device is out of alignment, and they can’t get the pin to release the points even after time’s run out.  This is at Avondale, CO, and they’re trying to get a new Siemens ACS64 into the test center.)

Let me give you an example from my Copper River & Northwestern Railway.  My layout is based on a proto-freelanced version of the historic CR&NW between Cordova and Kennicott, Alaska, set in present day.  As a modern day heavy ore hauler running through remote territory, the line is controlled by CTC.  Here’s the diagram for a stretch of track between the CTC control points (CP) at West Strelna and Moose Lake.  Between these CPs will be two sets of intermediate signals – one at Chokosna and one at Kuskulana.   Just short of the Chokosna intermediates is an uncontrolled switch into a gravel loading area.


In this view, nothing’s currently in the block, and the gravel spur switch is locked.  The signals are pretty normal – control points showing stop, intermediates showing clear (green) or approach (yellow) and set up for only a single approach block (meaning you only get one restricting signal before a stop, rather than several such as a sequence of advance approach, approach, stop).

Now, let’s suppose that the Chitina Local has tied up in the gravel spur for lunch.  Let’s also suppose that a loaded ore train – OTK1 (Ore Transport, Kennecott Mine, First Section) – is headed down from the mines, and has been cleared southbound all the way through Strelna.  Running on high greens, the OTK1 crew is thinking, “Great, we’ll make it back to Cordova in time for an early quit and dinner!”


Forgetting to call the dispatcher, the local crew goes ahead and prepares to re-enter the main line.   OTK1 has already passed the Chokosna intermediates on a green, since at that point the switch hadn’t been unlocked or thrown yet.  Since there are no signals between it and the unlocked switch, there’s no way to for OTK1’s crew to know the switch is being thrown in front of them.   Since it’s an uncontrolled switch, even the dispatcher doesn’t know until it’s too late.


With 6000 trailing tons and running at a track speed of 40mph, OTK1’s crew has no time to respond when they round the corner and see the open switch ahead of them.  They dump the air, but it’s too late.  If they don’t come off the rails going into the industrial spur, they’ll meet the local head-on.


Fortunately, it’s all in N scale.  The crews involved get a few demerits, a stern look from me and that’s about it.  Maybe I have to change a broken coupler on the front of one of the affected units.  Nobody gets hurt and everybody goes home to their families tonight.

The prototype doesn’t have that luxury, and because of that, the prototype signal departments invented the time lock – a device that sets a minimum time between when a crewman unlocks the switch and when they can actually change the points.  At the same time, the time lock device will shunt the track circuit, dropping surrounding signals as if there were a train in the block.  So let’s look at how that would work.


Here’s our local crew again, unlocking the switch.  Now it’s got a time lock on it, so there’s a delay.  When the timer starts, it also shunting the block, so the intermediates on each end drop to red.  If OTK1 is back north of CP Moose Lake, they’ll see a approach (yellow) signal and have time to bring the train to a controlled stop before the Chokosna intermediates.


If they’re past Moose Lake and on approach to Chokosna, they’ll get a clear block dropping to red right in front of them.  They’ll dump the air, but aside from a few flat wheels, they’ll be fine because the switch can’t be moved yet.


In the final case, OTK-1 has already passed the Chokosna intermediates.  The time lock is set so that any train in the block will have plenty of time to get from the signals to the gravel spur switch before the points can be moved.  OTK-1’s crew will have no idea the switch is even running on time.  However, the local crew will see the ore train passing before time runs out.  Once again, everybody’s safe.

Let’s say the gravel spur switch is set for 5 minutes.  By that time, OTK-1 should be safely most of the way through, or even out of the block.  Once that five minute runs out, the local crew can open the switch and be assured they won’t be meeting a highballing ore train head-on.



In the real world, manual switches within signalled territory are protected by devices called “time locks”.  The purpose of these is to prevent a switch from being opened in the face of an approaching train.

Modeling a Time Lock

We all have our focus in the hobby – those details that fascinate us and that we put untold hours into getting just right.  For me, it’s signals and control systems, and how they can be used to support better operating sessions.  Time locks on manual switches are extremely common on the prototype, but I’ve never seen one modelled.

The CKT-TIMELOCK is simple circuit to model an electrical time lock that I developed to implement proper time locking on my CR&NW.  It serves two purposes – more faithfully duplicating prototype signalling practices, and slowing down my switch crews.  My layout is reasonably small to start with (two decks @ 24ft. x 14ft., roughly), and so I need every element I can get to both add switching challenges and slow crews down.

The module is designed to sit between the control switch (generally a toggle on the fascia) and the switch machine itself.  By intercepting the control signal and doing some logic on it and a “lock” input, I could duplicate the prototype behaviour quite easily using a small microcontroller.   By adding a relay and shunt resistor, it would also be able to shunt the required block and drop the signals.

It effectively works by keeping the switch “normal” for as long as the keyswitch is locked, regardless of the input toggle.  Once an operator turns the keyswitch to “unlock”, the CKT-TIMELOCK will close the relay, shunting a 2k-ohm resistor across the rails, and start flashing an LED to let the user know that it’s running on time.  (The exact amount of time is user-programmable through a dip switch, ranging from 5 seconds to four minutes.)  Once time runs out, the LED will go solidly on and the control input from the direction toggle (probably mounted on the fascia) will be passed through to the switch machine.

When reversing the process, the device will not un-shunt the track and “lock up” the switch again until the keyswitch is turned back to “lock” and the points are returned to normal.  That prevents crews from leaving open switches around the layout, even though they’ve moved on.

The module is designed to be compatible with both our MRServo machines and with Circuitron Tortoise(tm) machines, as it has both a logic-level output and a bipolar 12V driver.  It should be adaptable to most slow motion machines, but those are the two that I specifically know will work.

Here’s a demo video of the CKT-TIMELOCK in action:

While my Copper River & Northwestern is still heavily under construction, I thought I’d share a few photos of the Chokosna spur switch that I used in the example above.  (The control panel is only temporary, until the fascia is installed.)

CR&NW Chokosna SwitchTemporary control panelCKT-TIMELOCK attached to a MRServo-2

Protect your crews – install time locks today!


  1. I just read and watched the demo on your electric lock interface an excellent idea. Two things though
    1 Is there a way for the crew to “wave the five and open the switch” after talking to the dispatcher??

    2 if you could develop or is there a way to interface these with say caboose hobbies ground throws.

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