While it's possible to run trains automatically on a layout by relying on accurate measurement and knowing exactly how fast your trains are moving, in practice you need feedback about where your trains are to make automation reliable, otherwise positions will drift  due to all sort of variations and inaccuracies.  There are several ways to detect a train on a track including optical detectors, switches, magnets and by measuring a voltage drop on a section of track.  However in most of these scenarios there is no information on exactly what has been detected, a bit like your neighbourhood fox tripping you PIR activated lights around the house where what you really want is to know when a person is there. As I wanted to know which loco was where on my layout, I went for a Railcom based system because I think this is the only way to do that right now. This allows a Railcom enabled decoder to send its ID and direction of travel back to a feedback detector module and then to the controller and finally to the control software.

The best bang for your buck in Railcom feedback detection is probably the Digikiejs DC5088RC which has 16 detectors on it for about £100-ish and that's what I went for they are easy for a geek like me to setup - you just plug them into your laptop taking care to only c9onnect them via USB isolator if you want to test them while the DCC system they are connected to is on.

Here you can see my APT-E (ID 152) showing up in the Digikeijs software:

The next question is how many feedback detectors are needed and where do they need to be? Here's my plan (with thanks to Iain Morrison for getting me started on this):

You can see I have 5 feedback detectors giving me detection across a maximum of 80 sections of track.  You can also see I have two reverse loop modules and I'll discuss those in another post, when they are installed and working.

I am using iTrain and this works by using a combination of detection, accurate speeds for trains and recording  the lengths of all the sections of track and points that make up a layout.  So when a train is detected iTrain can predict when it will enter the next detection section and use this to set points and stop trains in a station and so on.  However it is not precise -  Say there is a 1% margin of error on a 2m length of track. By the time a train comes to the end of that section you could be 2cm out, so having more detectors where trains are likely to brake and stop reduces this error.  For example in my 3m long station I'll assign 3 detectors , two short ones at either end of the platform for stopping depending on direction and a longer section to connect them.  Precise positioning is also important for shunting and uncoupling and that's another thing I plan to automate later on.

iTrain has a concept of a block that mirrors how real railways work where a block can only be occupied by one train (where a train is made up of one or mor locos plus other rolling stock) at a time.  Each block can have many feedback detectors but does not include points, so just one way in to the block and one way out or none if it's a siding.   As an  example in my main station  I have a block for every platform each with 3 detectors in.

It's pretty simple if a little tedious to describe blocks and feedbacks in iTrain but then I need to make that real by making changes to my layout:

• Wire up the feedback detectors to my track,
• Connect them via Loconet (Lnet in Roco speak) back to the DCC controller (Z21)
• Enter the details of the feedback detectors in iTrain.
• Test, Test, Test

That and splitting my single bus setup into a track plus accessory bus configuration required a lot of rewiring. Part of that was to add track power to each outside rail of all my points as I planned to fully insulate them from the adjacent tracks as I may want to add detection over points at a later date. This means the only things I have directly connected to the track bus are the detectors themselves - all power to detected sections,  point rails and frogs flows through one of these units..

 so I sort of have 7 buses on my layout! One for each of my 5 feedback detectors , the track bus and the accessory bus.  I am aware this might be overkill but it works.  It's also worth noting that both rails must be gapped between one detector and the next so that the common rail  (pink in my diagram) gets power from the same detector that's doing the detecting  

Of course drawing diagrams is easy so here's the spaghetti under my layout..

I guess I could have invested in a more Technicolor wiring scheme but the sticky labels I have attached to the wires are good enough for me.

I know I do now have a real mess of wires like my DC friends and some of the runs of dropper wires are longer than usual as the black wires all need to go to one of the five detectors.  However, the difference is that I now know exactly where my trains are as we'll see next time.

Finally thanks to my new friends Rich at YouChoos and James at DCC Train Automation who have been really helpful