The repair bay
Describe the fault in your own words — “it stalls on the points”, “the back wagon keeps coming off”, “everything shuts down” — and the matching faults rise to the top, each with the checks to make in order.
Twenty faults listed. Describe the problem above to narrow it down.
Almost always a feeder: solder a dropper to that length of rail rather than relying on the joiners. If the whole layout is dead, work back to the command station one district at a time.
Clean the axles, re-lubricate lightly — a smear of plastic-safe grease on gears, one drop of oil on bearings — and run it in gently for twenty minutes each way. Cracked split-axle gears are a known failure on some older models and are usually available as spares.
A pair of feeder wires soldered to that length of rail, dropped to the bus. Five minutes, and that spot never troubles you again.
Best: power the frog, switching its polarity with the point motor's spare contacts or an automatic frog juicer. Also good, and cumulative: fit a keep-alive capacitor, and add pickups so every wheel collects current. See powering turnouts.
Wheels first, metal wheels on the wagons second, CV2 third. Track cleaning is the least effective of the four and the one most people do first.
Ease the kink by relaying that joint with staggered rail ends; open a tight gauge by loosening the fixings and re-pinning; clear the flangeway with a scalpel. See laying track properly.
New wheelsets are a few pounds and cure most of these outright. Add weight low and central; bend the trip pin up with the tool made for it, or snip it off if you never use magnetic uncoupling.
Free off the dragging vehicle, weight the fleet consistently, and — if it keeps happening — ease the curve or run shorter trains through it.
Shim the coupler box up or swap to a coupler with an offset shank; ease the vertical transition with a long, springy sub-roadbed rather than a hinge.
Standardise on one coupler across the fleet. It is the single most worthwhile afternoon of fitting work a layout ever gets.
Correct the polarity, clear the obstruction, or re-cut the gap. Then do the quarter test across that area to confirm the protection still trips as it should.
Gap both rails at both ends and feed the isolated section through an auto-reverser (a DPDT switch on DC). Full explanation in reverse loops.
CV29 decides which is in use.CV19 from a previous double-heading session.CV1, CV17, CV18 and CV29. Reading is non-destructive and tells you exactly what the decoder thinks it is.CV19 is zero unless you meant it to be in a consist.Re-write the address, letting the command station handle the long-address arithmetic. If it is confused beyond that, write 8 to CV8 for a factory reset and start again. See the CVs worth knowing.
CV29, so they respond to the DCC waveform's offset.CV29, and often a setting in the command station too).Unique addresses, DC mode off, and a card in each box recording the number.
A keep-alive capacitor cures nearly every case, and improves slow running as a side effect. See sound and keep-alive.
Treat a flickering lamp as an early warning: it is showing you the interruptions that will soon stall the motor. Clean, add pickups, fit a keep-alive.
Clear the mechanism, add a CDU for solenoid motors, or replace with a slow-motion motor which pushes gently and stalls harmlessly — see powering turnouts.
Heavier bus wire (16–14 AWG) and a feeder to every length of rail. See bus and feeder wiring.
Clean the old lubricant out with a cotton bud, re-grease sparingly with plastic-safe grease, and replace any cracked gear. Bedding-in often quietens a new model considerably by itself.
Add weight low inside the body if there is room; shorten the train; double-head; or reduce the gradient. See gradients.
Before diagnosing anything in detail, do these. They take fifteen minutes and settle a surprising share of faults outright.
The temptation, with a train that will not run, is to clean the track and re-solder a feeder and adjust the coupler at once. Then it works, and you have learned nothing — so the next time it happens you start from scratch. One change, one test, every time.
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Whether it is DC or DCC; whether it happens with other locomotives at the same spot; and what changed just before it started. Answer those three unprompted and you will usually get the right diagnosis in one reply.
Fall back on the method rather than the symptom. Every fault on a model railway lives in one of four places, and two swaps tell you which.
Run a different locomotive over the same piece of track. If it also misbehaves, the track or the wiring is at fault. If it is fine, the first locomotive is.
Run the same locomotive somewhere else. Faults that follow the model are mechanical or electrical inside it; faults that stay put belong to the layout.
Try it light-engine, then with two wagons, then with ten. A fault that only appears under load is adhesion, drag or gradient — never electronics.
Faults that vanish at speed are pickup faults; faults that appear at speed are geometry — a curve, a kink or a check rail.
Between those four swaps you can place any fault on the layout in about five minutes, and then the relevant page on this site — track, locomotives, wiring — will tell you what to do about it.