Passing loop: how two trains pass each other
Benjamin | TrackDrawer.com · published 4 November 2026
A passing loop is a second track alongside a single-track line, with a turnout at each end. One train waits in the loop while the other runs past on the main line. You need one left-hand and one right-hand turnout, two compensating curves, two make-up pieces and, between the turnouts, as much straight track as your longest train is long.
Crossing and overtaking
On a single-track line, two trains heading towards each other cannot get past. The passing loop solves that: the first train runs into the loop and stops, the second runs through, then the first sets off again. That is called crossing. If both trains run in the same direction, a slow goods train can let a faster one overtake in the same way.
On a continuous run it works just the same. An oval with one passing loop is the smallest layout on which you can run two trains in turn: one goes round, the other waits. Put a platform along both tracks and you have a small through station. How that differs from a station where the tracks end at buffers is covered in terminus or through station. And a fiddle yard or hidden staging under the layout is, in essence, a row of passing loops out of sight.
How to build a passing loop
On a straight, from left to right:
- A turnout whose diverging route points to the side of the loop.
- Behind that route a compensating curve. It cancels the turnout angle so that the loop runs parallel, at the track spacing of your system.
- On the straight side, next to the compensating curve, a make-up piece so that both tracks stay the same length.
- Straight track in both tracks, as long as your train.
- The same in mirror image: make-up piece and compensating curve, then the second turnout.
Watch the hand of the turnouts. Seen from the points of the first turnout, it diverges to the left. The second turnout lies with its points facing the other way and, seen from its own points, diverges to the right. A passing loop therefore needs one left-hand and one right-hand turnout. A crossover between two parallel tracks needs two of the same hand. There is more on this in left, right and symmetrical turnouts; why the compensating curve ends at exactly the right distance is explained under track spacing.
How long is a passing loop?
A turnout with its compensating curve takes up a fixed length along the straight. Two of these groups back to back give the shortest passing loop you can lay, but there is no room for a train between them yet. The table shows the standard turnout with its compensating curve for each system in our catalogue.
| System | Turnout + compensating curve | Length along the track | Track spacing | Two groups together |
|---|---|---|---|---|
| Märklin C track | 24611 + 24224 | 360.1 mm | 77.5 mm | 720 mm |
| Märklin K track | 2262 + 2232 | 325.0 mm | 64.6 mm | 650 mm |
| Märklin M track | 5203 + 5206 | 359.8 mm | 77.4 mm | 720 mm |
| Roco geoLine | 61140 + 61128 | 384.7 mm | 76.5 mm | 770 mm |
| Piko A track | 55220 + 55219 | 470.0 mm | 61.9 mm | 940 mm |
| Fleischmann Profi | 6170 + 6138 | 399.9 mm | 63.3 mm | 800 mm |
| Kato Unitrack H0 | 2-840 + 2-270 | 375.0 mm | 74.6 mm | 750 mm |
| Hornby OO | R8072 + R606 | 335.2 mm | 66.7 mm | 670 mm |
| Tillig TT | 83332 + 83112 | 265.8 mm | 35.0 mm | 532 mm |
| Minitrix | 14951 + 14914 | 158.3 mm | 33.6 mm | 317 mm |
| Fleischmann piccolo | 9170 + 9136 | 222.6 mm | 29.3 mm | 445 mm |
| Kato Unitrack N | 20-220 + 20-160 | 249.0 mm | 32.8 mm | 498 mm |
| Märklin Z | 8562 + 8591 | 220.5 mm | 25.1 mm | 441 mm |
Add the length of the train that has to wait. Take four vehicles of 250 mm, the standard coach TrackDrawer uses for H0: that is 1 m. With C track the passing loop becomes 720 + 1000 = 1720 mm long, with Piko A track 1940 mm. In N, with four 136 mm wagons, Minitrix comes to 317 + 544 = 861 mm. Allow a little extra: the waiting train must stand completely clear of the point where the two tracks are still too close, or the passing train will brush against it.
Which baseboard fits a loop with a passing loop?
For each system the Auto-designer searched for the smallest baseboard on which it lays a closed loop with at least one passing loop, using a typical drawer of track (three left and three right turnouts, four compensating curves, four make-up pieces, straights and curves for every radius) and a 50 mm edge margin. The depth is always the smallest on which a closed loop fits.
| System | Depth | Smallest width | First variant | Pieces | Track |
|---|---|---|---|---|---|
| Märklin C track | 90 cm | 160 cm | Station with two platforms on R1, plus 2 sidings | 31 | 5.32 m |
| Märklin M track | 70 cm | 145 cm | Station with two platforms on Industriekreis, plus 2 sidings | 30 | 4.98 m |
| Märklin K track | 75 cm | 140 cm | Station with two platforms on Industriekreis, plus 2 sidings | 28 | 4.64 m |
| Roco geoLine | 90 cm | 165 cm | Station with two platforms on R2, plus 2 sidings | 30 | 5.58 m |
| Piko A track | 90 cm | 185 cm | Station with two platforms on R1, plus 2 sidings | 32 | 6.18 m |
| Fleischmann Profi | 85 cm | 165 cm | Station with two platforms on R1, plus 2 sidings | 36 | 5.95 m |
| Kato Unitrack H0 | 90 cm | 190 cm | Station with two platforms on R370, plus 2 sidings | 43 | 6.54 m |
| Hornby OO | 90 cm | 160 cm | Station with two platforms on R1, plus 2 sidings | 36 | 5.54 m |
| Tillig TT | 70 cm | 120 cm | Station with two platforms on R01 | 32 | 3.15 m |
| Minitrix | 55 cm | 85 cm | Station with two platforms on R1, plus 2 sidings | 34 | 2.72 m |
| Fleischmann piccolo | 55 cm | 100 cm | Station with two platforms on R1, plus 2 sidings | 26 | 3.00 m |
| Kato Unitrack N | 60 cm | 115 cm | Station with two platforms on R216, plus 2 sidings | 34 | 3.72 m |
| Märklin Z | 45 cm | 85 cm | Station with two platforms on R1, plus 2 sidings | 26 | 2.61 m |
A "station with two platforms" has a passing loop on both sides of the oval. For every system in the table the designer found a loop with a passing loop at that smallest depth with this drawer. A board 10 cm deeper makes it no narrower anywhere. The differences in width come mainly from the length of turnout and compensating curve: Märklin K track manages with 140 cm, while Piko A track and Kato Unitrack H0 need 185 and 190 cm. For Märklin C track the results per board size are in how much room for C track.
Passing on a curve or with Y turnouts
If your straight is too short, put the passing loop partly on the curve with curved turnouts. In our catalogue, Märklin C (24671 and 24672, from R1 to R2), K (2268 and 2269), M (5141 and 5142) and Z (8568 and 8569) among others have them. If they are in your collection, the Auto-designer uses them too. Fleischmann piccolo has one as well (9174 and 9175), but in our catalogue its outer road does not end on a circle; you lay that one yourself.
A second option is the Y turnout, which splits the track symmetrically: both trains take an equally gentle diverging route and neither runs straight. Piko A track (55226) and Hornby (R8076) have one. A passing loop made from two Y turnouts looks calm and is especially common on toy track such as Duplo.
Waiting without the loco moving off
Electrically a passing loop is no problem: it connects a track to itself in the same direction of travel, so there is no short circuit as with a reversing loop. The waiting train does have to stay put, though.
- Analogue control. All tracks get the same voltage, so the waiting loco moves too. Make a section of the loop, a little longer than the loco, switchable on its own: on two-rail you break one rail with insulated rail joiners, on Märklin three-rail the centre conductor. With the switch off, the loco stands still.
- Digital control. Every loco has its own address and stops when you want. No isolation is needed, unless you want feedback to tell you whether the loop is occupied.
Passing loops in TrackDrawer
Tick "Several trains at once" or "Station and shunting" in the Auto-designer and it looks for variants with passing loops: "Oval with passing loop", "Station" (a passing loop and a siding) and "Station with two platforms" (a passing loop on each side). It lays them with the turnouts, compensating curves and make-up pieces from your own collection, at the track spacing of your system. If you draw yourself, the check warns when two tracks are closer than the NEM spacing.
Enter your track and let the Auto-designer find a loop with a passing loop that fits your baseboard.
