Rolling Stock Maintenance

By Graham Oberst


The following recommendations will, with a few simple checks and adjustments, improve your rolling stock performance.  Following these recommendations will greatly reduce the frequency of uncouplings and derailments, which are a source of frustration on the best run railroads!


Wheel Standards.


Wheel Material.  Most rolling stock kits are supplied with plastic wheels, either a one-piece moulded plastic wheelset or plastic wheels pressed onto a metal axle.  The disadvantage of plastic wheels is that they readily pick up dirt and spread it round the track, resulting in poor electrical contact.  Metal wheels, which are usually supplied with the more expensive kits, do not suffer from this problem.  Replacement metal wheelsets are available from manufacturers such as Kadee, NorthWest Short Line and LifeLike Proto 2000 (Walthers).  Although the cost of metal wheels is about $3.50 per car their use is highly recommended.  Not only do they help to overcome dirty track problems, they also make cars more free-rolling.


Wheel Gauge.  The gauge of each wheelset should be checked using a National Model Railroad Association (NMRA) HO Standards Gauge.  This check is performed by placing the wheel flanges in the notches in one side of the gauge.  If the wheels are out of gauge they can be adjusted, unless the wheelset is one-piece moulded plastic.  Adjustment is done by carefully pushing or pulling one wheel on the axle until the wheels are in gauge.


Cleaning Wheels.  Dirt build-up on plastic wheel treads is clearly visible as a grey deposit.  The most effective way of removing this build-up is with a small screwdriver.  Turn the car upside down and hold the screwdriver against one wheel tread while turning the opposite wheel with your thumb, letting the blade scrape off the dirt.  Repeat this process with each wheel and give the wheel treads a final cleaning with a plastic-compatible solvent such as Goo Gone.  Regular cleaning with Goo Gone will keep wheels clean and will make it unnecessary to scrape off the dirt.


Wheel Wobble.  When a car wobbles as it rolls down the track it is usually a sign that one, or more, of the car’s wheels is not properly centred on the axle.  Turn the car upside down and spin each wheelset in turn to identify the defective wheel or wheels.  The only remedy for an off-centre wheel is to replace the defective wheelset.


Truck Standards.


Truck Play.  The truck frame is, in most cases, attached to the car body bolster with a small screw or a push-fit plastic pin.  The truck attachment should allow sufficient lateral (side-to-side) and longitudinal (end-to-end) play to accommodate any unevenness in the track.  If the truck attachment is too tight the car will tend to derail on uneven track or when passing through switches and sharp curves.  Conversely, if the attachment is too loose the car will tend to wobble.  The correct amount of play can be determined by trial and error.


Axle Pockets.  Most cars are equipped with trucks having plastic frames, with conical pockets in the side-frames for the needlepoint ends of the axle.  If the assembled car does not roll freely, it may be due to excess material in one or more of the axle pockets.  The excess material can be cleaned out by using a purpose-made tool such as the Exxact Socket, available from Reboxx, or the Truck Tuner, available from MicroMark Tools.  When these tools are inserted in the truck frame and rotated they remove any excess material and form an exact 60 degree axle pocket for the axle points, resulting in a free-rolling car.


Coupler Standards.


Coupler Types.  The horn-hook couplers supplied with most inexpensive kits are usually discarded and replaced with one of the more reliable and more realistic knuckle couplers which, in most cases, are a drop-in replacement.  For many years this meant one of the Kadee line of couplers, with the No.5 coupler being the most commonly used.  As Kadee’s patents expired a number of other companies started to make knuckle couplers and similar products are now available from a number of manufacturers such as Accurail, Bachmann, McHenry and LifeLike Proto 2000 (Walthers).  However, none of these manufacturers offers the same range of products as Kadee and, as a result, Kadee couplers are still regarded as the standard for HO scale. 

The different makes of knuckle coupler all function in a similar manner and each will couple readily with other makes of coupler, so it is not necessary to use only one make of coupler on all your rolling stock.  Scale-size couplers, which have a smaller coupler head, have been introduced recently by some manufacturers.  The smaller size of these couplers makes correct mounting height even more important for reliable operation.


Coupler Height.  The NMRA has established standards for coupler height since variations in coupler height from car to car can result in cars uncoupling on uneven track.  Coupler height can be checked quickly and easily by using a coupler height gauge, such as the Kadee No.205.  The gauge is placed on the track adjacent to the end of the car and will show immediately if the coupler is at the correct height.  Also, performing a “roll-by” inspection of passing cars against a light background will give a good idea of which cars need closer inspection. 

If the height gauge shows that the coupler is too low, it’s height can be raised by adding a fiber washer (or washers) between the truck frame and the body bolster.  Kadee makes two thicknesses of washer for this purpose, .010 thick (Kadee No.209) and .015 thick (Kadee No.208).  If the coupler is too high it may be possible to correct the problem by filing a small amount of material off the body bolster or the truck frame, or both.  If this approach will not correct the problem it will be necessary to insert a shim of suitable thickness between the coupler box and the car body.  If this is not possible it will be necessary to replace the coupler with one that has an offset shank.

Whether the coupler is too high or too low, a certain amount of trial and error will be required to arrive at the correct height.


Trip Pin Height.  The curved trip pin that hangs below the coupler must also be set at the correct height.  If the pin is set too low it can snag on switches, crossings and grade crossings and cause derailments.  If the pin is too high uncoupling magnets, located between the rails, may be unable to open the knuckle and uncouple the car.  (Since uncoupling magnets are not currently used on the club layout a trip pin that is set too high will not cause any problems.)  Trip pin height is also checked using the Kadee No.205 coupler height gauge.  The gauge is placed on the track adjacent to the end of the car and the trip pin should clear the small plate on the gauge, located between the rails.  The pin can be bent up or down, as required, using a pair of needle-nose pliers.  However, this task is made easier by using a pair of purpose-made trip pin pliers such as those offered by Kadee (No.237) and MicroMark Tools.


Coupler Swing.  To ensure that cars couple easily and can negotiate switches and sharp curves, couplers should swing freely from side to side and should come to rest on the car centerline.  All makes of knuckle coupler have a spring, or springs, to center the coupler and, in most cases, the couplers will swing freely and self-centre as assembled.  However, on cars where the coupler cover is attached with a small screw, care must be taken not to over-tighten the screw as this can deform the cover and cause the coupler to bind, restricting side-to-side movement and self-centreing action.  Adding a small amount of powdered graphite lubricant, such as Kadee Greas-em (Kadee No.231) during coupler assembly will help to ensure free movement of the coupler.      


Car Weight.


NMRA Standard.  Correctly weighted cars ride better through switches and on uneven track, making them less likely to derail.  The NMRA has established a recommended practice (RP 20.1) for the weight of a car based on its length.  The formula uses an initial weight plus additional weight per inch of car length.  For HO scale cars the initial weight is 1oz (ounce) and the additional weight is 0.5oz per inch of car length up to a maximum weight of 6oz.  As an example, for a 50ft car, which is about 7 inches long, the recommended weight is the initial weight of 1oz plus 7 x .5oz = 3.5oz for a total weight of 4.5oz.  Car weights do not have to be exact but, as previously stated, a correctly weighted car will ride better.


Adding Weight.  Car kits usually include a weight, or weights, and some cars are close to the recommended weight straight from the box, while others are too light.  A small postal or kitchen scale works well for measuring car weight.  Measurement should be made before the car is fully assembled to determine how much additional weight is needed.  If additional weight is required several options are available.  The easiest solution is to use lead weights with peel-and-stick backing, made by A-Line and others.  These weights are easy to apply and come in .25oz and .5oz sections.  Hex nuts and flat washers, readily available in hardware stores, can also be used and can be attached to the car floor with double-sided tape or plastic-compatible adhesives such as two-part epoxy or thick cyanoacrylate adhesive (CA).  Whichever type of weight is added it should be centred in the car, both side-to-side and lengthwise, for proper tracking.  Weights can be added easily to closed cars such as boxcars, reefers and covered hoppers but are more difficult to add to open cars such as flatcars, gondolas and open hoppers.  In these cases the weight can be hidden under the car floor or in a load.



Useful References from Model Railroader.


  • HO Scale Magnetic Couplers.  (July 2000, p.58)
  • Back to Basics; Fine-tuning HO Knuckle Couplers.  (May 2002, p.98)
  • Back to Basics; Car Weighting.  (February 2003, p.56)
  • 10 Weeks to a Better Layout.  (August 2004, p.52)
  • 5 Ways to Improve Your Freight Cars.  (November 2004, p.64)
  • MR Project Illustrated; Rolling Stock Test Stand.  (August 2005, p.122)



Graham Oberst, October 2005.

Revised:  November 2007.  



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