Train derails at the turnout: causes and fixes
Benjamin | TrackDrawer.com · published 30 October 2026
If your train derails at a turnout, it is almost always one of five things: the blades do not close properly, the joint between turnout and track has a step or a kink, the wheelset is not to NEM dimensions, the turnout is not laid flat, or the curve right behind the turnout is too tight. First check whether it is always the same turnout and the same vehicle, and in which direction it goes wrong. That alone usually gets you halfway to the cause.
First: where and when does it happen?
Run slowly over the turnout and watch from close by, or film it with your phone at eye level. Look for three things:
- Direction of travel. If the train runs facing the blades (from the toe of the turnout towards the two routes), the problem is usually at the blades. If it runs from the two routes towards the toe, look first at the frog and the check rails, or at a turnout set against the train.
- Which vehicle. If the same coach or locomotive always comes off, the vehicle is the problem. If everything comes off, it is the turnout or the track.
- Which wheel. The first wheel to leave the rail shows you where to look.
The checklist
- The blades do not close properly. A flange that slips between a half-open blade and the stock rail splits the train: the front axle goes left, the rear one right. Causes include dirt or a grain of ballast between blade and rail, glue or paint on the pivots, or a point motor that does not push the blade fully home. Clean the turnout, remove loose grains and check that the motor gets enough power. An electric motor that only throws halfway often points to a weak supply or stiff mechanism.
- A step or kink at the joint. If the turnout sits slightly higher or lower than the adjoining track, or at the foot or top of a gradient, the flange climbs over the rail head. Lay a ruler across the joint: it should sit on both rail heads without a gap. A change from level to gradient does not belong on or right next to a turnout.
- Loose or bent rail joiners. A rail joiner that does not grip the rail foot firmly allows a small offset in the rail. You can feel it sideways with a fingertip. Press it home or replace it.
- Wheelset not to NEM dimensions. The distance between the inside faces of the wheels decides whether a wheel is guided cleanly past the frog. If that distance is too large or too small, the flange hits the nose of the frog or climbs the check rail. Check every suspect wheelset with an NEM gauge for your scale and move the wheels on the axle if you can. Old stock with deep flanges can also bump into the fixings of a finer rail profile.
- Turnout not flat or under stress. A turnout screwed down too tightly onto a baseboard that is not flat will twist. One blade or one rail ends up slightly higher, and a light wheel lifts. Loosen the screws: if it runs then, the turnout was under stress. Lay it on a flat surface and screw it down without force.
- Too tight a reverse curve right behind the turnout. If the diverging route is followed straight away by a curve the other way, the train runs through an S-bend. Long coaches stand there with their buffers at an angle to each other and push each other off the rails. Leave at least one coach length of straight between two opposing curves.
- Couplings that catch in an S-bend. Even without buffer contact, couplings can be pulled off centre in an S-bend. Check that all couplings are at the same height, sit firmly in the NEM pocket and swing freely.
- Too much speed. A turnout is a place with a short curve and a gap in the rail. What works at low speed can just fail at full speed, especially on the diverging route. Slow down over turnouts.
- Dead frog. On two-rail the frog of many turnouts is not powered. A short locomotive with few pickup wheels stutters or stalls there. That is not a derailment, but a sudden stop can push a propelled train off the track. Clean wheels and rails help, as does a locomotive that picks up on more axles.
Turnout geometry per system
How tight the diverging route is depends on the system. The table shows the standard turnout from our catalogue, with the turnout angle and the radius of the diverging route.
| System | Turnout | Turnout angle | Diverging radius | Smallest curve in the system |
|---|---|---|---|---|
| Märklin C track | 24611 | 24.3° | 437.5 mm (R2) | R1 360 mm |
| Märklin K track | 2262 | 22.5° | 424.6 mm | Industriekreis 295.4 mm |
| Roco geoLine | 61140 | 22.5° | 502.7 mm | R2 358 mm |
| Piko A track | 55220 | 15° | 907.97 mm | R1 360 mm |
| Fleischmann Profi | 6170 | 18° | 647 mm | R1 356.5 mm |
| Hornby OO | R8072 | 22.5° | 438 mm | R1 371 mm |
| Minitrix | 14951 | 24° | 194.6 mm (R1) | R1 194.6 mm |
| Kato Unitrack N | 20-220 | 15° | 481 mm | R216 mm |
| Märklin Z | 8562 | 13° | 490 mm | R1 145 mm |
On most systems the diverging route of the turnout is more generous than the smallest curve, so the turnout itself is rarely the tightest spot. On Minitrix the standard turnout is as tight as the smallest curve. Put an R1 curve the other way straight after the diverging route there and you get a sharp S. With long stock, look above all at what comes after the turnout.
How TrackDrawer helps
TrackDrawer checks your plan for whether everything joins up, whether any rails overlap, whether there is a height difference at a joint and whether a gradient starts too abruptly. For an S-bend with less straight between two opposing curves than the length of a standard coach (250 mm in H0, 136 mm in N), you get a tip to put a straight in between. The pattern of a turnout with its own matching curve does not count, because the manufacturer designed it that way. That takes part of the causes out of the plan before you drive in a single screw. What remains are the things you only see on the real layout: dirt, wheelsets and a baseboard that is not flat.
Frequently asked questions
Why does only my longest coach derail?
Because long coaches swing out the most. In an S-bend right behind a turnout their buffers and couplings are furthest off centre. Look first at the track behind the turnout and put a straight between the opposing curves there.
Why does it only go wrong in one direction?
Each direction loads different parts. Facing the blades, what matters most is whether the blade closes properly. From behind, it is mainly the frog and the check rails, and whether the turnout is set right: running through a turnout from behind while it is set against you forces the blades aside and often ends in a derailment.
Should I screw my turnouts down?
On a permanent layout, yes, but without stress. Only screw down once the surface is flat, and tighten just enough that the turnout cannot shift. If a train derails after screwing down and not before, loosen the screws first.
Does a slimmer turnout help?
Often, yes, because the diverging route is more generous. On Märklin C track the slim turnout 24711 has a diverging radius of 1114.6 mm against 437.5 mm for the standard turnout, but it is also longer, so plan first whether it fits.
Enter your track and let the Auto-designer lay out a plan that TrackDrawer checks straight away, or read how to build a reliable layout with old track and the right track spacing.
