Feedback and detection: knowing where your train is
Benjamin | TrackDrawer.com · published 13 November 2026
Feedback tells the command station or a computer where a train is. A detector in the track gives a signal when a train is over or on a particular section, and a feedback module such as an s88 passes it on. There are two kinds: a momentary report that says once that a train is passing, and occupancy detection that reports for as long as a section is occupied. For a train that stops on its own, or a fiddle yard that knows which road is free, you need occupancy detection.
Momentary or occupied
The difference decides what you can do with the report.
- Momentary report. A train passes a point and the detector gives a short pulse. Good for a level crossing that has to close, a signal that should go to danger behind the train, or a counter. The detector does not know whether anything is still on the track.
- Occupancy detection. As long as a vehicle is on a given section, the report stays on. The command station or program therefore also knows that a stationary train is still there. That is the basis for block signalling and automatic running.
Three ways to detect
Contact track (occupancy on three-rail)
On Märklin three-rail both outer rails are normally the return. In a contact track one outer rail is electrically separated from the rest. When a wheelset runs over it, the metal wheels and axle connect that rail to the other one: the report comes on. It stays on as long as a metal wheelset is standing on it, even when the train has stopped. That makes the contact track an occupancy detector, and you can make it as long as you like by separating several pieces in a row on the same side.
Switching track and reed switch (momentary)
A switching track has a small switch operated by the loco's pickup shoe, or a similar mechanism. It gives a short pulse at the moment the loco passes. A reed switch works with a small magnet under the loco or wagon: the contact in the track closes briefly as the magnet goes over it. Both only report the passing, and only vehicles with the right part trigger a report.
Current sensing (occupancy on two-rail)
On two-rail you separate one rail of a section with insulated rail joiners and feed that section through a detector. If something draws current in that section, the detector reports it occupied. Locos draw current, and so do lit coaches. An ordinary wagon with insulated wheels draws nothing and stays invisible. Many modellers therefore fit a few wagons, and certainly the last vehicle of every train, with wheelsets that carry a small resistor, so that those are detected too.
The detectors in our catalogue
The catalogue has these pieces with a detection function per track system:
| System | Piece | Kind |
|---|---|---|
| Märklin C track | 24995 contact track 94.2 mm (2 pieces) | occupancy |
| Märklin C track | 24994 switching track 94.2 mm; 24194 curved R1 15°; 24294 curved R2 15° | momentary |
| Märklin K track | 2295 contact track set 2 x 90 mm | occupancy |
| Märklin K track | 2299 switching track 90 mm; 2229 and 2239 curved 15° | momentary |
| Märklin M track | 5104 contact track 180 mm | occupancy |
| Märklin M track | 5146 switching track 90 mm; 5147 curved R1 15° | momentary |
| Fleischmann piccolo | 9115 contact track 55.5 mm | feedback |
Other systems in the catalogue have no separate detection piece. With Kato Unitrack, for example, you separate a section with insulated joiners and connect a detector there. How to lay such a break is covered in track sections and isolation.
The s88 bus
A detector is only a contact. To reach the command station you need a feedback module. The best-known bus for that is s88: modules daisy-chained on one cable, each with a fixed number of inputs. In our catalogue the three feedback modules each have 16 inputs: the 60881 s88 AC, the 60882 s88 DC and the 60883 Link s88, which connects the modules to the command station. Choose the version that suits your track system and command station; which one that is, their manuals tell you.
The more modules in the chain, the longer the cable. Keep the chain short and do not run the cable right next to the track power wires over long distances, because interference produces false reports. Other manufacturers use buses of their own; more on that in DCC and Märklin Motorola.
Where to put detectors
A few rules for a plan you can automate later:
- At least one occupancy detector per block. A block is a section of track where only one train may be at a time. Without a detector nobody knows whether it is free.
- Two detectors per block for clean stops. Many computer programs work with an entry detector that makes the train slow down and a stop detector just before the end. That way every train stops at the same spot whatever its speed.
- A detector of its own for every road in a fiddle yard. Then you know which road is free without looking. More about hidden storage in fiddle yard and hidden sidings.
- Leave room. A block has to be longer than your longest train, plus the braking distance up to the stop detector.
What you can do with feedback
Even without a computer, feedback is useful: the command station shows which tracks are occupied, or switches a signal or level crossing. With a computer program such as Rocrail or JMRI it goes further: trains run from block to block, stop at an occupied block and set off when it clears. The command station drives the locos; the program decides based on the reports.
Frequently asked questions
Does feedback work on analogue too?
Yes, but without a command station. A contact track or switching track can switch a signal, a level crossing or a relay directly. That is how an analogue train stops at a red signal: the signal cuts power to the section in front of it. You only need an s88 module when the reports have to go to a command station or computer.
How many feedback modules do I need?
Count your detectors: one or two per block, plus one per road in a fiddle yard. Divide that by the number of inputs per module, 16 for the modules in our catalogue, and round up. Keep a few inputs free for whatever gets added later.
Feedback in TrackDrawer
With the digital layer (Pro) for Märklin C track, every contact or switching track in your plan (24994, 24995, 24194 and 24294) automatically gets an input on a feedback module from your collection, 16 per module. The wiring diagram numbers the detection tracks C1, C2 and so on and shows, per module, which input belongs to which track. TrackDrawer derives the blocks from signals and turnouts; the detection tracks in a block are its detectors. In the Rocrail export the first detector of each block becomes the entry detector, and the JMRI export lists the detectors with their addresses. If a feedback module is missing, it shows up under shortages.
Lay your detection tracks in the app and let TrackDrawer build the inputs, the blocks and the wiring diagram.
