Locomotive stalls on the frog
Benjamin | TrackDrawer.com · published 1 November 2026
If a locomotive always stalls at the same spot in a turnout, it is almost always running over a dead frog and, at that moment, has no wheel left that picks up current. It happens on two-rail with short locomotives that pick up on only a few axles. The fix is a combination of clean wheels and rails, pickup on more wheels and, where possible, a frog that carries current itself.
Why the frog can be a dead spot
At the frog the two inner rails of a turnout cross. On two-rail one rail is positive and the other negative. If the frog were simply made of metal and joined to both rails, positive would meet negative: a short circuit. Manufacturers solve this in two ways.
- Insulated frog. The frog is plastic, or the metal part is isolated on both sides. It is safe and needs no wiring, but there is a short stretch without power in the route.
- Live frog. The frog is metal and takes the polarity of the rail the turnout is set for. There is no dead spot, but the polarity has to switch with the turnout.
A two-rail loco keeps running as long as at least one wheel on the left picks up current and one on the right does too. Over an insulated frog, part of the pickup on one side drops out. If the loco has only two pickup wheels close together on that side, both can sit on the dead part at the same time. The motor cuts out and the loco coasts a fraction further, or stops. With a longer wheelbase or pickup on more axles, there is always a wheel on live metal.
What the frog looks like and why the wheels have to bridge a small gap there is also covered in train derails at the turnout; that article mentions the dead frog only briefly as a cause. This one is about the electrical side.
Two kinds of frog in the catalogue
In our catalogue the difference is clearest with Peco Streamline. The same turnout comes in two versions there: Insulfrog (insulated frog) and Electrofrog (live frog). The geometry is identical; only the frog and the rail code differ.
| Size | Insulfrog (Code 100) | Electrofrog (Code 75) | Diverging radius | Turnout angle | Length |
|---|---|---|---|---|---|
| small | SL-91 right, SL-92 left | SL-E191 right, SL-E192 left | 610 mm | 12° | 185 mm |
| medium | SL-95 right, SL-96 left | SL-E195 right, SL-E196 left | 914 mm | 12° | 219 mm |
| large | SL-88 right, SL-89 left | SL-E188 right, SL-E189 left | 1524 mm | 12° | 258 mm |
Because the shape is the same, you can swap one version for the other in a plan without any change. Electrically it is a different choice: the Electrofrog needs more wiring, the Insulfrog has a dead spot. For other systems the frog type is not in our catalogue. Check the documentation of your own turnouts.
Three-rail: hardly an issue with Märklin
On Märklin C, K and M track both outer rails together form the return. All wheels pick up that return, and the traction current comes from the centre conductor through the pickup shoe under the loco. Positive and negative cannot meet at the frog, so the rails there can be metal and joined together. That is why three-rail does not have the dead frog problem in the same way.
If a Märklin loco still stalls on a turnout, look at the pickup shoe and the studs. A worn, dirty or bent shoe loses contact first where the studs are laid out differently, such as in a turnout. Clean the shoe and the studs, and check that the shoe still springs freely. Note: Trix C track has the same shape as Märklin C track but is two-rail. The two-rail rules in this article apply there.
Checklist for a loco that stalls
- Is it always the same loco? If only your smallest shunter stalls and everything else runs through, the problem is that loco's pickup, not the turnout.
- Clean wheels and rails. On a dead spot every wheel counts. A dirty wheel that goes unnoticed elsewhere is, on the frog, exactly the wheel that should have been picking up. Clean the railheads before and after the frog and the switch blades in particular. How to clean track and wheels thoroughly is covered in train runs jerkily.
- Look at the blades. On many turnouts with an insulated frog, the blades and the rails behind them get their power through the contact between blade and stock rail. Dirt, paint or ballast glue on those contact faces makes a whole section of the turnout dead, not just the frog.
- Don't crawl. At a crawl the loco has no momentum to roll across the dead spot. Taking the turnout a little faster often helps.
- More pickup. Locos that pick up on all wheels, or through the tender as well, run far better over an insulated frog. On some models you can add extra pickups; that is fine work.
- A stay-alive capacitor on digital. If you run digital, a stay-alive capacitor on the decoder can keep the loco moving for a brief moment without track power. It bridges a dead frog, but it does not cure dirt.
- A live frog. If none of this helps, choose turnouts with a live frog, or power an existing frog if the turnout is suited to it.
Wiring a live frog properly
A metal frog removes the dead spot, but it asks three things of you.
- Insulated rail joiners beyond the frog. The frog changes polarity. The two rails that continue beyond it must therefore be separated from the rest of the layout with insulated rail joiners, or you get a short circuit the moment the turnout is thrown.
- Reliable switching. Don't rely only on the contact between blade and stock rail for the polarity. A changeover contact on the point motor, or an electronic module that switches the frog polarity automatically on digital, is more reliable. The principle is the same as on a reversing loop: the module detects the short and flips the polarity before the command station trips.
- Wheels in gauge. With a live frog, blade and stock rail of opposite polarity sit close together. A wheelset that is out of gauge can touch the wrong rail with the back of a wheel and cause a short.
If you run through a live-frog turnout from the trailing end while it is set against you, the loco moves from one polarity onto the other: that is a short as well. Always set the turnout correctly, or let a point motor do it.
How TrackDrawer helps
TrackDrawer does not work out which turnout has a dead spot; that depends on the model you buy. What you do see in the parts list is every turnout in your plan by catalogue number, so with Peco Streamline you know straight away whether you need Insulfrog or Electrofrog. On two-rail the check warns you if your plan contains a reversing loop anywhere, naming the two pieces between which you have to gap both rails. How to choose turnouts for a plan is explained in turnouts: left, right and symmetrical.
Frequently asked questions
Why does my loco only stall in one direction?
Because the pickup wheels cross the dead spot differently in each direction, for example when the loco has a bogie without pickup at one end. A blade that makes poor contact in one position also only matters in that position.
Can I power an Insulfrog later?
That depends on the model and means working on the turnout itself. If you have many short locos, choose a live frog from the start on a new layout.
Enter your track and let the Auto-designer lay out a plan; the parts list then shows exactly which turnouts you need.
