Block Detection for signalling and automation


Hi Folks

When blocking a section of track for signalling and automation, is it necessary to isolate and detect each turnout, or can the turnout be part of the approach block?  I can see how it might be 'better' to detect the turnout, but does it actually matter when it comes down to it?  I'm thinking the answer is 'isolate them for signalling, but don't bother for automation', but I'd like confirmation.  If I don't need separate detection in staging, it saves me a significant number of detection circuits.


Blair Smith


Hi Blair;

It depends (don't you love that answer?) 

Turnouts for industry spurs and other similar connections to "track other than main track" do not need to be individually detected. The tracks they lead to should be gapped and not detected. You don't want a boxcar at the Joe's Widget Factory loading dock to trip the detector. 

Given that, it is good practice to make turnouts lined away from the main track force the block to show occupied. I do this by placing a 10k ohm resistor between the frog and the diverging stock rail. 

As far as turnouts at the ends of passing sidings, or at the end of double track, or crossovers between multiple track, it depends. For the signaling on my layout (ABS/APB) turnouts are part of the block they are adjacent to, depending on signal configuration at that location. For CTC, these turnouts are isolated. It seems like a good to isolate turnouts for automation so that you can prevent throwing them while occupied.

Tim Rumph
Lancaster, SC


Hi Tim

That's more along the lines of what I was looking for in an answer, thanks.

I was already planning on dealing with industry, interchange, and house tracks that way.  As for turnouts for automation, it's a good point - but really, the automation should be looking at least 1-2 blocks ahead of the train and aligning turnouts appropriately, unless there are extenuating circumstances.  If a turnout is in a block, it shouldn't need to be thrown once occupied.

Looks like someone's going to have to push some more $$$ in to the detection budget.  Ouch!  I think I'm heading for somewhere around 100 blocks; between the detection, and the reporting equipment, and JMRI, I'll be busy for a while.  Good thing it's a hobby!  I think I'll gap it all for future detection, even if I then incorporate multiple sections into one block initially. Easier to separate out and detect separately if needed.

Thanks again



Blair there is a great new book out about signaling. Guide to signals and interlocking by Dave abeles. It pretty much tell you everything you need to know. Kalmbach publishing issued the book.

Don Vollrath

The easy way for signaling is to make the turnout and points part of the mainline signal block leading up to it. Both of the tracks leading away from the frog should be isolated and start the next signal blocks. If one of them is a siding it does not need to be separately sensed. Your signaling logic may want to include the turnout position.