This information is divided into 3 Categories; Common Causes, Less Common Causes, and You’ve got to be Kidding!

Common Causes

Track out of gauge
Track not level (BOTH Ways) along the length and side to side. Note that properly installed track may be super-elevated but this requires detailed attention to Rolling Stock.
Mismatched rail code causing height variation.
Rail joint too large.
Track obstructions (on or near) Nails, scenery, buildings bad weld or joint.
Turnouts (not level, improper gauge or wheel clearance at guard rails or frog.
Curve radius too tight for locomotive or rolling stock
Improper Wheel Gauge
Coupler height (coupler and trip pin).
Kink at rail joint (easy to do with flex track)
Turnout points not firmly contacting stock rail (both sides)
Turnout points not shaped right to fit stock rail at top of points
Wheels both in gauge, but slipped toward one end of the axle so that, when tracking, neither axle’s wheels are directly behind/in front of their counterparts on the next axle;
Points rails not sharp enough;
Couplers that aren’t really compliant or suited for matching;
Couplers that aren’t mounted suitably for the curvature radius being used
“S” curve in track
Turnout not reset to proper position. (left switched to siding or spur).
Turnout switch points dance around, not held in place by ground throw or switch machine.
Turnout directly after curve causing a S curve.
Turnouts too tight for longer locomotives or rolling stock.
Loose ballast in a turnout.
Wheel sets not properly set in trucks.
Coupler ‘brake hose’ too low.
Sloppy kinky laid track and old BB and Roundhouse cars hastily built with defects overlooked.
Speed..Yup that “S” curve between switches taken at Marc one or light speed.
Truck mounted couplers.
Trucks to tight won’t allow trucks to turn freely-not sloppy by being to loose.
Long wheel base engines and cars on sharp curves.
Excessive Speed
Everything on these lists going forward, double it for going backwards. Pushing a string even if everything is near-perfect.
Stiffly sprung diaphragms on passenger cars, operated on sharp radius curves.
Long cars being coupled to short cars because they have a different amount of overhang on curves and the longer car will pull the short car off of the track

Less Common Causes
Trucks are deformed wheels don’t all sit flat on track.
The Steam loco is connected to the tender by the cable but the bar is disconnected.
A piece of a loco detail or rolling stock has come off and is on track or causing wheels to lift.
Vehicles or livestock on track
Switch machine too close on curve causing loco plow or other body part/detail to hit.
Un-coupler magnet to high or not flat/level
Poorly adjusted truck movement on rolling stock
Steam loco lead or trailing trucks not sprung properly
Steam loco floating / sprung drivers not properly sprung (too strong or weak)
Diesel loco wheelsets just a tad out of gauge. The flanges should be centered in the NMRA gauge slots, not just sorta fitting into the slots
Sudden Grade Changes
Axles not seated properly in the trucks’ bearing cones;
Sags or humps leading through the points and toward the diverging leg of a turnout;
Frog poorly designed or made allowing wheel drop or improper tracking;
Wrong tire and flange profile for the NMRA-compliant turnout
Lack of weight on leading trucks
Sprung trucks taking an uneven set from track work and not leveling back out.
Narrow guardrail gaps and tight spacing between frog point and guard rail inside edge.
Truck swivel impeded by under body details

You’ve GOT to be Kidding!
Main higher than the sidings. Where the grade change for the siding occurs “inside” the switch. Very bad
Problems at the ends of modules where the foam shrank and lowered all the track except at the end that sits on plywood
Steam engine to tender connector that lifts the engine in a curve
Hanging wires from motor/engine
Unbalanced weight on engine
Improperly installed pilot trucks on steam locomotives!
Rail gaps that become too wide due to changing weather conditions